The contribution of snacks (defined as any food eaten between the main meals) to daily diet quality and health is a somewhat controversial subject at present. This chapter aims to evaluate the evidence for the effects of snacking behavior (e.g., snack frequency, size, energy density, and other parameters) on overall diet quality and weight-related outcomes, taking into account also some other aspects that can motivate people to snacking. Regardless of the age of the study population, snacking frequency can have a positive, negative, or null impact on diet quality and weight status. However, the choice of specific snack foods is associated in the expected way with diet quality and body weight outcomes—the worse the nutritional profile of snacks and the higher their energy density, the worse the quality of the overall diet and weight-related outcomes. Considering the impact of snacking on diet quality and health status, we should also consider additional factors such as the food environment (e.g., snacking availability) and the timing of snacks. Moreover, there is a need for scientific consensus of what constitutes a snack and snacking.