Note: This paper is a pre-print of: Danny Roest, Ali Mesbah and Arie van Deursen. Regression Testing AJAX Applications: Coping with Dynamism. In Proceedings of the 3rd International Conference on Software Testing, Verification and Validation (ICST’10), Paris, France. IEEE Computer Society, 2010. There is a growing trend to move desktop applications towards the web using advances made in web technologies such as AJAX. One common way to provide assurance about the correctness of such complex and evolving systems is through regression testing. Regression testing classical web applications has already been a notoriously daunting task because of the dynamism in web interfaces. AJAX applications pose an even greater challenge since the test case fragility degree is higher due to extensive run-time manipulation of the DOM tree and asynchronous client/server interactions. In this paper, we propose a technique, in which we automatically generate test cases and apply pipelined oracle comparators along with generated DOM templates, to deal with dynamic non-deterministic behavior in AJAX user interfaces. Our approach, implemented in CRAWLJAX, is open source and provides a set of generic oracle comparators, template generators, and visualizations of test failure output. We describe two case studies evaluating the effectiveness, scalability, and required manual effort of the approach.