Unit testing is a technique that receives a lot of criticism in terms of the amount of time that it is perceived to take and in how much it costs to perform. However it is also the most effective means to test individual software components for boundary value behavior and ensure that all code has been exercise adequately (e.g. statement, branch or MC/DC coverage). In this paper we examine the available data from three safety related software projects undertaken by Pi Technology that have made use of unit testing. Additionally we discuss the different issues that have been found applying the technique at different phases of the development and using different methods to generate those test. In particular we provide an argument that the perceived costs of unit testing may be exaggerated and that the likely benefits in terms of defect detection are actually quite high in relation to those costs.