Bilingualism is a complex construct, and it can be difficult to define and model. This paper proposes that the field of bilingualism can draw from other fields of psychology, by integrating advanced psychometric models that incorporate both categorical and continuous properties. These models can unify the widespread use of bilingual and monolingual groups that exist in the literature with recent proposals that bilingualism should be viewed as a continuous variable. In the paper, we highlight two models of potential interest- the factor mixture model and the grade-of-membership model. These models simultaneously allow for the formation of different categories of speakers and for variation to exist within these categories. Researchers can then conduct their analyses on either the categorical or continuous information, or a combination of the two, depending on which is most appropriate to address their research question. Conceptualizing bilingualism within the context of these more flexible models will help to advance theory and lead to a fuller and deeper understanding of bilingualism.