The question "What to Study in HCI" has two parts. First it asks how HCI researchers think about the research challenges they tackle: how do they decide what problems to engage with and how to study them? Second, the question also asks what is the subject of HCI: which challenges should researchers address and, ultimately, what makes us unique as a discipline? While there have been intermittent discussions on this topic in HCI, the present workshop emphasizes this question and explore some possible answers among a group of seasoned researchers. One reason is our belief that researchers can benefit from addressing these questions so as to develop their practical understanding (e.g., "tricks of the trade") of how to tackle the complexity of selecting "what to study". Second, we argue that researchers can benefit from thinking about the epistemological grounds upon which they base their everyday work, that is, thinking about what HCI is. The workshop results in publicly available key readings and position papers on "What to Study in HCI". Copyright is held by the author/owner(s).