During the last ten years the market for smartphones has grown drastically. Because of the current state of the market with different operating systems many smartphone applications need to be developed for several platforms. With this thesis, the goal was ultimately to contribute to the understanding of cross-platform development as a way of developing smartphone applications. React Native has been evaluated as a framework with which development is done for both Android and iOS using the same code. To study React Native as a framework, a smartphone application for Android and iOS was developed at an Uppsala based IT-company with expertise in web services, smartphone applications, and online gaming. Furthermore, performance tests and user tests were conducted in which React Native was compared to native applications and applications developed using Xamarin (similar cross-platform development framework owned by Microsoft). It was evident that using the same code for both Android and iOS was time saving. However, the performance tests results showed that applications developed with React Native did not perform as well as the native and Xamarin versions. Leading to the conclusion that choice of framework when developing cross-platform applications need to take into consideration performance, development time, and programming language preference.