Application Programming Interface (API) development is growing in popularity and because of that, it is more important than ever to develop high-quality APIs. There is often a rush to build APIs and therefore their design can suffer consequently. This may have a negative impact on the companies and consequently a poor user experience. Different “first” methodologies decide the approach when it comes to developing APIs. The purpose of this study is to investigate what API-first methodology has for value when developing APIs. This is done by developing two APIs where one is using the API-first approach while the other makes use of Code-first. This thesis is using a qualitative method of semi-structured interviews to collect data, with the focus to highlight the differences between the APIs developed. Specifically, the differences in test efficiency, user experience, and API correctness.  The conclusion from the study points out that by using API-first the user experience gets a higher quality in terms of documentation, test efficiency, and accuracy in the business requirements. Also, it provides a suggestion for when and why a company should use it. The findings suggest that when the project is big and complex, then API-first is a good option to consider. The continuous dialog between developer and client is why it is such an approving method to use. The hope is that the findings of this thesis are an indicator that API-first is a significant method to use and that more companies consider it when developing APIs.