Developing interest is a powerful support for deeper learning. The presence of even some interest beneficially affects individuals’ attention and memory, as well as their motivation and meaningful engagement. In this chapter, we expand on previous descriptions of the relation between interest and its development as conceptualized in the Four-Phase Model of Interest Development (Hidi & Renninger, 2006; Renninger & Hidi, 2016). We explain that interest has a physiological basis, and therefore is universal – meaning that all persons, regardless of age or context, can be supported to develop at least some interest in topics to be learned. We describe how and when interest is likely to develop. We review findings which provide evidence that the structure of tasks and activities, as well as interactions with other people, may be helpful to interest development, and also that when these supports are mismatched with the learner's phase of interest, they may constrain or impede interest development. We point to interest as a determinant of learners’ understanding, effort, and feedback preferences, and the coordination of their phase of interest development with their abilities to set and realize goals, feel self-efficacy, and self-regulate. We conclude by identifying some open questions concerning the process of interest development and learning.