The longstanding debate between dimensional and categorical approaches to reading difficulties has recently been rekindled by new empirical evidence and developments in theory. At the heart of the categorical perspective is the tenet that dyslexia is a taxon, a grouping of cases that can account for both intra-group similarities and inter-group differences. As developmental dyslexia is characterized by a diverse constellation of symptoms with multiple underlying risk and protective factors, the key question in dyslexia research has shifted from “What is dyslexia?” to “How many taxons or subtypes of dyslexia are there?” The primary objective of this chapter is to consider methods that can be used to objectively define these groupings, starting with the current practice of defining subtypes of readers using normative scores with pragmatically dened cut-offs, the “Quadrant Analysis” approach, and progressing towards more theoretically sound and statistically rigorous procedures. We review and test several candidate approaches that can be readily adapted to realistic conditions that are problematic for Quadrant Analysis. Specifically we propose a method that can be used to identify subgroups in the bivariate case when the two indicator variables are correlated. We conclude by evaluating the strengths and weaknesses of this and other methods and include implications for their future application toward identifying and validating putative dyslexia taxons.