I always seem to spend ten minutes trying to find these snippets of code on the Internet when I need them. For reference . . .

To trigger a macro on a change in range, load the procedure below into a sheet module. It will fire when a change is made to the range A1:A5.

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
    If Not Intersect(Target, Range("A1:A5")) Is Nothing Then
        Your Code Here
    End If
End Sub

To trigger a macro on a change in the sheet, load the procedure below into a sheet module. It will fire when a change is made to any cell on the sheet:

Private Sub Worksheet_Change(ByVal Target As Range)
    Your Code Here                
End Sub