Stammering is a speech disorder affecting approximately 1% of the worldwide population. It can have associated impacts on daily life, such as loss of confidence in social situations and increased anxiety levels (particularly when speaking to strangers). Work exploring the development of digital tools to support people who stammer (PwS) is emerging. However, there is a paucity of research engaging PwS in the design process, with participation being facilitated mainly in testing phases. In this paper, we describe the user-centered design, development and evaluation of StammerApp, a mobile application to support PwS. We contribute insights into the challenges and barriers that PwS experience day-to-day and reflect on the complexities of designing with this diverse group. Finally, we present a set of design recommendations for the development of tools to support PwS in their everyday interactions, and provide an …