Despite the substantial expansion in vocabulary research since the 1980s, we still know very little about how vocabulary develops over time and what factors influence this development. This methodological overview discusses key issues and considerations in vocabulary breadth growth assessment to help advance research in this area. The report begins by discussing general issues in vocabulary assessment such as sampling rate and the effect of cognates. This is followed by an overview and an evaluation of common vocabulary breadth tests. The report ends with recommendations for choosing vocabulary tests for vocabulary growth research.