While the use of test-driven development as a debugging, pedagogic, and analytical methodology for objectoriented and procedural systems is well documented, it is a relatively unexplored and informal practice within the paradigm of source transformation. This paper describes a test-driven approach to the specification and evaluation of source transformation programs through rule-by-rule and type-by-type unit testing. We introduce the Transformation Engineering Toolkit for Eclipse (TETE), a test-driven framework centered around a simple yet flexible infrastructure for automatically and non-invasively unit testing subtransformations, application strategies, and grammar types specified in the TXL source transformation language.