Geoscientists analyze and integrate spatial and temporal information at a range of scales to understand Earth processes. Despite this, the concept of scale is ill defined and taught unevenly across the K-16 continuum. This literature review focuses on two meanings of scale: one as the magnitude of the extent of a dimension and the other as a relationship between objects or events. We review 42 papers from science education and discipline-based education research (DBER) literature on students' conceptions related to one or both meanings of scale. Analysis of this prior work reveals a broader (though still limited) research base on domain general concepts of scale as magnitude and scant research on scale as a relationship. Learners begin reasoning about spatial and temporal magnitudes categorically by working with scales based on standard units and nonmetric values, such as body length. Concepts of scale magnitudes outside human experience are nonlinear. Facility with fractions and proportional reasoning are positively associated with the ability to reason about scale as a relationship. Two constructs from the psychological literature, structure mapping and the category adjustment model, offer theoretical accounts for these findings. We borrow a typology from the psychological literature to frame common geoscience instructional models in the context of spatial and temporal scale and suggest how instructors might facilitate students' reasoning about scale models. We identify a number of avenues for possible future research, including a critical need to understand how conceptual understanding of scale develops across the K-16 continuum.