The calyptra is a small, maternal gametophyte structure that covers the apex of the offspring sporophyte and is critical for the transition from seta formation to capsule differentiation in many moss taxa. It has been hypothesized to function by (1) providing a mechanical constraint that coordinates the development of the presumptive capsule, (2) secreting hormones that influence sporangium differentiation, (3) physically protecting the undifferentiated capsule region, (4) preventing water loss from the sporophyte apex, and/or (5) lowering the rate of sporophyte transpiration. This review explores the experiments that have tested these hypotheses and the evidence that has been found to support or refute them from scientific literature dating back to the late 1700's. Across mosses,... calyptrae come in a wide array of shapes and sizes, which may correlate with aspects of morphology, habitat, and function. The overarching objective of this paper is to renew discussion about and research into this small, but important moss organ.