diff --git a/WGneu/Helpers/Utils.cs b/WGneu/Helpers/Utils.cs
index f27a78f..4d181cf 100644
--- a/WGneu/Helpers/Utils.cs
+++ b/WGneu/Helpers/Utils.cs
@@ -61,5 +61,9 @@ namespace WGneu.Helpers {
                 UseShellExecute = true,
             });
         }
+
+        public static bool MgNrExists(AppDbContext ctx, int mgnr) {
+            return ctx.Members.Find(mgnr) != null;
+        }
     }
 }
diff --git a/WGneu/Helpers/Validator.cs b/WGneu/Helpers/Validator.cs
index 315faa3..c5e3e39 100644
--- a/WGneu/Helpers/Validator.cs
+++ b/WGneu/Helpers/Validator.cs
@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Windows.Controls;
+using WGneu.Models;
 
 namespace WGneu.Helpers {
     static class Validator {
@@ -29,11 +30,11 @@ namespace WGneu.Helpers {
             { "423", Array.Empty<string[]>() },
         };
 
-        public static ValidationResult CheckNumericInput(TextBox input, bool optional) {
-            return CheckNumericInput(input, optional, -1);
+        public static ValidationResult CheckNumeric(TextBox input, bool optional) {
+            return CheckNumeric(input, optional, -1);
         }
 
-        private static ValidationResult CheckNumericInput(TextBox input, bool optional, int maxLen) {
+        private static ValidationResult CheckNumeric(TextBox input, bool optional, int maxLen) {
             string text = "";
             int pos = input.CaretIndex;
             for (int i = 0; i < input.Text.Length; i++) {
@@ -60,7 +61,7 @@ namespace WGneu.Helpers {
         }
 
         public static ValidationResult CheckPlz(TextBox input, bool optional) {
-            CheckNumericInput(input, true, 4);
+            CheckNumeric(input, true, 4);
             if (optional && input.Text.Length == 0) {
                 return new(true, null);
             } else if (input.Text.Length != 4) {
@@ -235,21 +236,51 @@ namespace WGneu.Helpers {
         }
 
         public static ValidationResult CheckLfbisNr(TextBox input, bool optional) {
-            var res = CheckNumericInput(input, true, 7);
-            if (!res.IsValid)
+            var res = CheckNumeric(input, true, 7);
+            if (!res.IsValid) {
                 return res;
-            if (optional && input.Text.Length == 0)
+            } else if (optional && input.Text.Length == 0) {
                 return new(true, null);
-            if (input.Text.Length != 7)
+            } else if (input.Text.Length != 7) {
                 return new(false, "Betriebsnummer zu kurz");
+            }
 
             // TODO
 
             return new(true, "Not implemented yet");
         }
 
-        public static ValidationResult CheckUstIdInput(TextBox input, bool optional) {
+        public static ValidationResult CheckUstId(TextBox input, bool optional) {
             return new(false, "Not implemented yet");
         }
+
+        public static ValidationResult CheckMgNr(TextBox input, bool optional, AppDbContext ctx, Member m) {
+            var res = CheckNumeric(input, optional);
+            if (!res.IsValid) {
+                return res;
+            } else if (optional && input.Text.Length == 0) {
+                return new(true, null);
+            }
+
+            int nr = int.Parse(input.Text);
+            if (nr != m.MgNr && Utils.MgNrExists(ctx, nr)) {
+                return new(false, "Mitgliedsnummer wird bereits verwendet");
+            }
+
+            return new(true, null);
+        }
+
+        public static ValidationResult CheckPredecessorMgNr(TextBox input, bool optional, AppDbContext ctx) {
+            var res = CheckNumeric(input, optional);
+            if (!res.IsValid) {
+                return res;
+            } else if (optional && input.Text.Length == 0) {
+                return new(true, null);
+            } else if (!Utils.MgNrExists(ctx, int.Parse(input.Text))) {
+                return new(false, "Ein Mitglied mit dieser Mitgliedsnummer existiert nicht");
+            }
+
+            return new(true, null);
+        }
     }
 }
diff --git a/WGneu/Windows/MemberListWindow.xaml b/WGneu/Windows/MemberListWindow.xaml
index 26e18f4..2a8dcdd 100644
--- a/WGneu/Windows/MemberListWindow.xaml
+++ b/WGneu/Windows/MemberListWindow.xaml
@@ -115,10 +115,12 @@
                 </Grid.ColumnDefinitions>
 
                 <Label Content="MgNr.:" Margin="10,10,0,0" Grid.Column="0"/>
-                <TextBox x:Name="MgNrInput" Margin="0,10,0,0" Width="48" Grid.Column="1" TextAlignment="Right" HorizontalAlignment="Left"/>
+                <TextBox x:Name="MgNrInput" Margin="0,10,0,0" Width="48" Grid.Column="1" TextAlignment="Right" HorizontalAlignment="Left"
+                         TextChanged="MgNrInput_TextChanged" LostFocus="MgNrInput_LostFocus"/>
 
                 <Label Content="Vorg.:" Margin="10,10,0,0" Grid.Column="2"/>
-                <TextBox x:Name="PredecessorMgNrInput" Margin="0,10,10,0" Width="48" Grid.Column="3" TextAlignment="Right" HorizontalAlignment="Left"/>
+                <TextBox x:Name="PredecessorMgNrInput" Margin="0,10,10,0" Width="48" Grid.Column="3" TextAlignment="Right" HorizontalAlignment="Left"
+                         TextChanged="PredecessorMgNrInput_TextChanged" LostFocus="PredecessorMgNrInput_LostFocus"/>
 
                 <Label Content="Vorname:" Margin="10,40,0,0" Grid.Column="0"/>
                 <TextBox x:Name="GivenNameInput" Margin="0,40,0,0" Grid.Column="1"
@@ -200,7 +202,8 @@
                 </Grid.ColumnDefinitions>
 
                 <Label Content="UID:" Margin="10,10,0,0" Grid.Column="0"/>
-                <TextBox x:Name="UstIdInput" Margin="0,10,10,0" Grid.Column="1" Width="120" HorizontalAlignment="Left"/>
+                <TextBox x:Name="UstIdInput" Margin="0,10,10,0" Grid.Column="1" Width="120" HorizontalAlignment="Left"
+                         TextChanged="UstIdInput_TextChanged" LostFocus="UstIdInput_LostFocus"/>
 
                 <Label Content="BetriebsNr.:" Margin="10,40,0,0" Grid.Column="0"/>
                 <TextBox x:Name="LfbisNrInput" Margin="0,40,10,0" Grid.Column="1" Width="64" HorizontalAlignment="Left" TextAlignment="Right"
diff --git a/WGneu/Windows/MemberListWindow.xaml.cs b/WGneu/Windows/MemberListWindow.xaml.cs
index fc2fe38..e2c7832 100644
--- a/WGneu/Windows/MemberListWindow.xaml.cs
+++ b/WGneu/Windows/MemberListWindow.xaml.cs
@@ -112,7 +112,7 @@ namespace WGneu.Windows {
             MgNrInput.Text = NextMgNr().ToString();
         }
 
-        private void MemberList_SelectionChanged(object sender, SelectionChangedEventArgs evt) {
+        private void MemberList_SelectionChanged(object sender, RoutedEventArgs evt) {
             RefreshInputs();
         }
 
@@ -165,6 +165,8 @@ namespace WGneu.Windows {
                 m = new();
 
             int newMgNr = int.Parse(MgNrInput.Text);
+            m.PredecessorMgNr = (PredecessorMgNrInput.Text == "") ? null : int.Parse(PredecessorMgNrInput.Text);
+            m.Predecessor = Context.Members.Find(m.PredecessorMgNr);
             m.Prefix = (PrefixInput.Text == "") ? null : PrefixInput.Text;
             m.GivenName = GivenNameInput.Text;
             m.FamilyName = FamilyNameInput.Text;
@@ -257,7 +259,7 @@ namespace WGneu.Windows {
             RefreshMemberList();
         }
 
-        private void Menu_Member_SendEmail_Click(object sender, EventArgs evt) {
+        private void Menu_Member_SendEmail_Click(object sender, RoutedEventArgs evt) {
             Utils.MailTo(((Member)MemberList.SelectedItem).Email);
         }
 
@@ -467,7 +469,15 @@ namespace WGneu.Windows {
         }
 
         private void InputTextChanged(TextBox input, bool optional, Func<TextBox, bool, ValidationResult> checker) {
-            var res = checker(input, optional);
+            InputTextChanged(input, optional, (tb, opt, ctx) => checker(tb, opt));
+        }
+
+        private void InputTextChanged(TextBox input, bool optional, Func<TextBox, bool, AppDbContext, ValidationResult> checker) {
+            InputTextChanged(input, optional, (tb, opt, ctx, m) => checker(tb, opt, ctx));
+        }
+
+        private void InputTextChanged(TextBox input, bool optional, Func<TextBox, bool, AppDbContext, Member, ValidationResult> checker) {
+            var res = checker(input, optional, Context, (Member)MemberList.SelectedItem);
             Valid[input] = res.IsValid;
             if (res.IsValid) {
                 if (InputHasChanged(input)) {
@@ -481,8 +491,16 @@ namespace WGneu.Windows {
             UpdateButtons();
         }
 
-        private void InputLostFocus(TextBox input, bool optional, Func<TextBox, bool, ValidationResult> checker, string? msg) {
-            var res = checker(input, optional);
+        private void InputLostFocus(TextBox input, bool optional, Func<TextBox, bool, ValidationResult> checker, string? msg = null) {
+            InputLostFocus(input, optional, (tb, optional, ctx) => checker(tb, optional), msg);
+        }
+
+        private void InputLostFocus(TextBox input, bool optional, Func<TextBox, bool, AppDbContext, ValidationResult> checker, string? msg = null) {
+            InputLostFocus(input, optional, (tb, optional, ctx, m) => checker(tb, optional, ctx), msg);
+        }
+
+        private void InputLostFocus(TextBox input, bool optional, Func<TextBox, bool, AppDbContext, Member, ValidationResult> checker, string? msg = null) {
+            var res = checker(input, optional, Context, (Member)MemberList.SelectedItem);
             if (!res.IsValid)
                 MessageBox.Show(res.ErrorContent.ToString(), msg ?? res.ErrorContent.ToString(), MessageBoxButton.OK, MessageBoxImage.Warning);
         }
@@ -528,13 +546,29 @@ namespace WGneu.Windows {
             UpdateButtons();
         }
 
+        private void MgNrInput_TextChanged(object sender, RoutedEventArgs evt) {
+            InputTextChanged((TextBox)sender, false, Validator.CheckMgNr);
+        }
+
+        private void MgNrInput_LostFocus(object sender, RoutedEventArgs evt) {
+            InputLostFocus((TextBox)sender, false, Validator.CheckMgNr);
+        }
+
+        private void PredecessorMgNrInput_TextChanged(object sender, RoutedEventArgs evt) {
+            InputTextChanged((TextBox)sender, true, Validator.CheckPredecessorMgNr);
+        }
+
+        private void PredecessorMgNrInput_LostFocus(object sender, RoutedEventArgs evt) {
+            InputLostFocus((TextBox)sender, true, Validator.CheckPredecessorMgNr);
+        }
+
         private void PlzInput_TextChanged(object sender, RoutedEventArgs evt) {
             InputTextChanged((TextBox)sender, false, Validator.CheckPlz);
             UpdatePlz((TextBox)sender, OrtInput);
         }
 
         private void PlzInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckPlz, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckPlz);
             UpdatePlz((TextBox)sender, OrtInput);
         }
 
@@ -543,7 +577,7 @@ namespace WGneu.Windows {
         }
 
         private void PhoneNrInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckPhoneNumber, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckPhoneNumber);
         }
 
         private void EmailInput_TextChanged(object sender, RoutedEventArgs evt) {
@@ -551,7 +585,7 @@ namespace WGneu.Windows {
         }
 
         private void EmailInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckEmailAddress, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckEmailAddress);
         }
 
         private void IbanInput_TextChanged(object sender, RoutedEventArgs evt) {
@@ -559,7 +593,7 @@ namespace WGneu.Windows {
         }
 
         private void IbanInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckIban, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckIban);
         }
 
         private void BicInput_TextChanged(object sender, RoutedEventArgs evt) {
@@ -567,7 +601,15 @@ namespace WGneu.Windows {
         }
 
         private void BicInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckBic, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckBic);
+        }
+
+        private void UstIdInput_TextChanged(object sender, RoutedEventArgs evt) {
+            InputTextChanged((TextBox)sender, true, Validator.CheckUstId);
+        }
+
+        private void UstIdInput_LostFocus(object sender, RoutedEventArgs evt) {
+            InputLostFocus((TextBox)sender, true, Validator.CheckUstId);
         }
 
         private void LfbisNrInput_TextChanged(object sender, RoutedEventArgs evt) {
@@ -575,7 +617,7 @@ namespace WGneu.Windows {
         }
 
         private void LfbisNrInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckLfbisNr, "Betriebsnummer ungültig");
+            InputLostFocus((TextBox)sender, true, Validator.CheckLfbisNr);
         }
 
         private void BillingPlzInput_TextChanged(object sender, RoutedEventArgs evt) {
@@ -584,7 +626,7 @@ namespace WGneu.Windows {
         }
 
         private void BillingPlzInput_LostFocus(object sender, RoutedEventArgs evt) {
-            InputLostFocus((TextBox)sender, true, Validator.CheckPlz, null);
+            InputLostFocus((TextBox)sender, true, Validator.CheckPlz);
             UpdatePlz((TextBox)sender, BillingOrtInput);
         }
     }