Although there exist a range of professional standards and guidelines for evaluating individuals from diverse linguistic and cultural backgrounds, there is a conspicuous absence of approaches that meet or satisfy the recommendations related to fairness and equity, particularly with reference to English learners (ELs). ELs are defined simply as anyone who is not a native English speaker and did not learn English as their only langauge. As such, ELs are typically bilingual due to the presence of at least one language other than the one they learned from birth. But bilingual does not always imply English as the second language as does EL. Likewise, an EL may be limited English proficient (LEP) or they may have become fluent English speakers over time. In either case, their current level of proficiency does not eliminate the fact that they were and always will be English learners. For these reasons, the term “EL” is preferred as it clearly denotes the type of multilingual individual being referred to in the present discussion regarding test performance. The measurement of cognitive abilities in ELs is based largely on a wide and highly disparate set of studies that have been conducted for over a century without any guidance from established or proposed theory. Despite the lack of consistency in various aspects of the empirical process, there has, nonetheless, emerged some basic principles upon which there is agreement and evidence. These principles include the variability of developmental differences in language which affects cognitive test performance and its increasing, proportional, and attenuating impact on tests as a function of the degree to which a given test requires and expects age-based developmental language proficiency and acculturative knowledge acquisition. The purpose of this paper is to provide a general review and critical discussion of the various studies conducted to examine the cognitive test performance of ELs and the extent to which they provide support for the these basic principles. In addition, an attempt is made to provide greater clarity on the various concepts, their definitions, and relationships to other variables relevant to the assessment of ELs as a way of fostering a more consistent and consensus-based understanding regarding the multiple factors arising from measurement of cognitive abilities in ELs and their significance for future research and practice. In this manner, the findings are ultimately distilled and coalesced into a cohesive and related set of empirically derived conclusions that together provide a foundation for the development and refinement of theory, a consistent platform and guide for conducting useful empirical investigations, and a framework by which recommendations for applying research into practice can be made so as to create evidence-based assessment (EBA). In addition to the historical and contemporary research, current approaches used in the evaluation of ELs are discussed relative to their advantages and limitations. Finally, guidelines for best practice are delineated as well as new developments and potential directions for future research and practice frameworks that may enhance our understanding and measurement of cognitive abilities in ELs that approaches the aspirational standards of fairness and equity in evaluation.