Automatically adding a tag to a deal based on set criteria

Automatically adding a tag to a deal based on set criteria

Hi all
I want to add a tag to deals based on whether (the margin is >x% *and* the amount is > £y) *or* (if a specific dropdown field is set to a specific value). 

Is it possible to do this via a workflow or similar? If so, can anyone point me in the right direction or let me know how?

Olly