In this chapter we describe a trend we have observed in 3rd-wave HCI research, which we are calling "peripheral practices research". This form of research consists of primarily qualitative studies of niche, unusual, marginalized and/or highly specialized communities of practice that result in implications for HCI outside of that community. We describe how peripheral practices research serves three critical functions within HCI: (1) It introduces a diversity of perspectives into the field; (2) It identifies new approaches to existing problems and challenges; (3) It serves as a defamiliarizing lens on existing norms and assumptions within the field. We survey a broad and diverse selection of studies that engage with peripheral practices, and discuss four specific cases in more detail. By giving this mode of HCI research a name, we hope to see even more studies that look outside of the classical HCI domain for new ideas, new perspectives, and new values around technology.