The crystallographic orientations of echinoderm skeletal elements can supplement standard morphological comparisons in the exploration of echinoderm evolution. At a coarse scale, many echinoderms share a crystallographic pattern in which c axes radiate away from the axis of pentaradial symmetry. Within this common pattern, however, c axes of different taxa can differ dramatically in their degree of variability, angles of inclination, and relationships to the external morphology of skeletal elements. Crystallographic data reflect a variety of taxon-specific influences and therefore reveal different information in different taxa. In echinoids, orientations of c axes in coronal plates correlate well with high-level taxonomic groupings, while c axes of apical plates record modes of larval development. In blastoids, c axes of radial plates have a structural interpretation, with the c axis oriented parallel to the orientation of the surface of the radial plate during its initial growth stages. In crinoids, c axes do not correlate with taxonomic group, plate morphology, or developmental sequence, but instead correlate with relative positions of skeletal elements on the calyx. Although their full potential has yet to be explored, the varied crystallographic patterns in echinoderms have been used to clarify skeletal structure, characterize developmental anomalies, and infer homologies of skeletal plates both within specimens and between groups. A axes are less constrained in their orientations than c axes and offer less promise of revealing novel paleobiological information.