Android apps interact with their environment extensively, which can result in flaky, slow, or hard-to-debug tests. Developers often address these problems using test doubles—developer-defined objects that replace a production class during test execution. Despite the fact that test doubles are widely used, there is limited understanding of how they are used in practice. To bridge this gap, we present an in-depth empirical study that aims to shed light on how developers create and use test doubles in Android apps. In our study, we first analyze a dataset of 1,006 apps with publicly available test suites to identify which frameworks and approaches developers most commonly use to create test doubles. We then investigate several research questions by studying how test doubles defined using these popular frameworks are created and used in a set of representative apps. Our results, based on the analysis of 2,365 test doubles that replace a total of 784 production classes, provide insight into the types of test doubles used within Android apps and how they are used. Our results also show that test doubles used in Android apps and traditional Java test doubles differ in at least some respect. Finally, our results show that test doubles can introduce test smells and even mistakes in the test code. In the paper, we also discuss some implications of our findings that can help researchers and practitioners working in this area and guide future research.