We describe a visual interactive framework that supports the computation of syntactic unifiers of expressions with variables. Unification is specified via built-in transformation rules. A user derives a solution to a unification problem by stepwise application of these rules. The software tool provides both a debugger-style presentation of a derivation and its history, and a graphical view of the expressions generated during the unification process. A backtracking facility allows the user to revert to an earlier step in a derivation and proceed with a different strategy.