Information system development (ISD) projects are an ever-growing field of project management (PM) with their unique features, and project failures in ISD are relatively common. In the broader context of PM, uncertainty is a studied, yet mercurial phenomenon. By contrast, uncertainty in ISD projects has received relatively little attention from scholars, and PM literature has not systematically focused on uncertainty in ISD from a viewpoint other than that of project managers. In order to understand uncertainties in ISD projects, we need to first understand the causes behind them, their effects on everyday ISD work, and share coping mechanisms utilized among industry professionals. In the context of ISD projects, we set out to explore what causes uncertainty, what are the effects of uncertainty, and how software industry professionals cope with uncertainty. We conducted eleven semi-structured interviews with a diverse range of ISD professionals, and analyzed the interviews using conventional content analysis. Our results extend and complement current knowledge on the causes, effects, and coping mechanisms of uncertainty, especially in the context of ISD. Additionally, we present practical considerations on how to implement our findings into ISD industry and education.