Testing is often considered as an expensive and uncontrollable process. Testing takes too much time, costs a lot more than planned, and offers insufficient conception of the quality of the test process. Therefore, the quality of the information system and the risks for the business can be difficult to determine. Many organisations realise that improving the test process can solve these problems. However, in practice it turns out to be hard to define what steps to take for improving and controlling the process, and in what order. The Test Process Improvement (TPI) model has been developed based on the practical knowledge and experiences of test process development. TPI offers a viewpoint in the maturity of the test processes within the organisation. Based on this understanding the model helps to define gradual and controllable test process improvement steps. In this paper the contents and the structure of the TPI model is introduced. Also some general aspects of the test process improvement and its challenges are discussed.