Grimmia plants are small, yellowish to blackish green and nearly always found on bare rock. The leaves are generally ovate‑lanceolate, imbricate when dry, have a strong, single costa, small, thick‑walled upper leaf cells and usually lack alar cell differentiation. Useful taxonomic features are found in the cross‑sectional shape of the leaves (keeled vs concave), leaf margins (plane, recurved, or incurved), lamina (unistratose, bi‑ or multistratose), and basal leaf cells (sinuate or straight‑walled; elongate, short rectangular or quadrate).

The Grimmia sporophyte has distinctive features that have been used to delimitate subgeneric (Loeske 1930, Jones 1933) or generic groups (Churchill 1981). Especially important characters are setae orientation (straight, flexuose, arcuate or cygneous) and capsule form (erect and symmetric vs. ventricose and asymmetric). The groups defined by these sporophytic characters appear to represent monophyletic assemblages, but nearly half the species are dioicous and rarely produce sporophytes.