We mainly study development velocity in agile teams in this dissertation. The concept of development velocity relates to the classical problem of time estimation in software development and software development planning. Building on previous literature as well as a case study, we explore and theorize the factors that cause ‘velocity flux’, i.e. fluctuations in development velocity through studying the relationship between development velocity and the rate of incoming customer feature requests. The aim of this study is to contribute to a better understanding of what causes velocity flux in agile development, and discusses the implications of the findings for research and practical implications for agile planning. As a result, we propose nine factors that cause velocity flux, and provide some strategies to overcome them in order to make a more effective sprint planning in agile teams.