In VBA module in "ThisWorkbook" capture the
SelectionChange event. If the address of the target is
the correct cell address, then execute code.

Example :

Sub Worksheet_SelectionChange (ByVal Target as Range)
If Target.Address = "$A$1" then
' put your code here
end if
End Sub
-----Original Message-----
How can I get a macro to run automatically only when a
specific cell e.g. A1 has its value changed?
Many thanks.
.