Dependent on the user name I want to restrict certain user from updating a certain value in a field i.e.if username A entered a value say "CAT" an error msg will pop-up however for username B if he entered "CAT" it is OK.

TQ in advance.

In the before update event of the control add in

If Username<>"UsernameA" then
Msgbox "Only Username A can edit this
cancel=true
else
end if

This code will not work but it gives you an idea... The code needs modifying depending on how you determine who the current user is.