Simon I copied this of the Access help file, hope it's of use:
You can use the Required property to specify whether a value is required in
a field. If this property is set to Yes, when you enter data in a record, you
must enter a value in the field or in any control bound to the field, and the
value cannot be Null. For example, you might want to be sure that a LastName
control has a value for each record. When you want to permit Null values in a
field, you must not only set the Required property to No but, if there is a
ValidationRule property setting, it must also explicitly state
"validationrule Or Is Null".
Note The Required property doesn't apply to AutoNumber fields.
