The importance of early vocabulary development to later reading comprehension has been well-established. However, there have been a number of criticisms that the assessments typically used to measure oral vocabulary knowledge do not adequately capture the complexity of this construct. This conceptual review works towards a more robust theoretical framework for vocabulary knowledge, focusing especially on the understudied dimension of vocabulary depth, which can be used to evaluate and design measures for early childhood learners. This framework is then used to review measures commonly used for preschool to 1st grade learners in the context of vocabulary interventions and observational studies. Recommendations are made for the use of existing measures and the design of future measures.