This commentary discusses some theoretical and methodological issues related to research on the spacing effect in second language acquisition research (SLA). There has been a growing interest in SLA in how the temporal distribution of input might impact language development. SLA research in this area has frequently drawn upon the rich field of cognitive psychology as a motivation for research and a context for the discussion of results. However, there are a number of nonconformities between these two fields, including how key constructs have been operationalized and measured. A better understanding of these conceptual divergences will allow SLA to advance with more systematic and robust research into the impact of input spacing on second language development. Copyright © 2017 Oxford University Press.