Just to expand a little on Boyd's excellent example, you can loop through all the controls. This assumes that you have a tag of "Req" on any field that is required.

[vba]
For Each ctl In Me.Controls
Select Case ctl.ControlType
Case acTextBox, acComboBox
If ctl.Tag = "Req" Then
If nz(ctl, "") = "" Then
MsgBox "This field is required!", vbCritical, "Required Field Missing"
ctl.SetFocus
Cancel = True
End If
End If
End Select
Next ctl

[/vba]