System dynamics validity consists of two very different types: structural and behavioral. A unique property of system dynamics method is that structure validity must precede behavior validity; the latter has no value without the former. We thus present structural tests and behavior pattern tests in this order and separately. Structure tests are further classified into direct and indirect structure tests. We place special emphasis on indirect structure tests. These are strong (special) behavior tests that can provide information on potential structural flaws. Since they combine the strength of structural orientation with the advantage of being quantifiable, indirect structure tests seem to be the most promising direction for research on model validation. We demonstrate two different validity testing software designed at Bogazici University to assist with indirect structure testing (SiS) and behavior pattern testing (BTS II). SiS software does automatic dynamic pattern recognition, indirect structure testing and pattern=oriented model calibration. Our behavior pattern testing (BTS II) software is also pattern-oriented. It estimates and compares the major dynamic pattern components that exist in the simulated and actual dynamic behaviors. *Supported by Bogazici University Rereacrh Fund no. 06HA305