A fundamental goal of etiologic stroke classification is to generate subgroups with discrete phenotypic, therapeutic, and prognostic features. Accurate stroke classification requires integration of multiple aspects of diagnostic stroke evaluation in a standardized manner. Diagnostic test findings can be simply organized into major etiologic groups to create a phenotypic subtype, or they can be reduced to a single causative subtype through a decision-making process. It is essential for a classification system to provide consistent results across different raters in different clinical settings. Comparability of subtype assignments is the key to valid communication of research results across the field. This article highlights important theoretical aspects of etiologic stroke classification and reviews major etiologic classification systems that have benefited from recent advances in etiologic stroke evaluation.