The character and significance of pigeonite was first demonstrated by H. H. Hess in 1941. Extensive subsequent studies are reviewed in regard to crystallographic, chemical, and paragenetic relations among the calcium-poor members of the CaSiO3 - MgSiO3 - FeSiO3 group of pyroxenes. Pigeonites of P21/c and C2/c symmetry comprise a link with Pbca hypersthenes and C2/c subcalcic augites, respectively, and constitute a vital key to our understanding of pyroxene relations. Recent structural and experimental thermal studies corroborate and refine Hess's views on the processes of subsolidus exsolution and inversion. The name "pigeonite" should be retained and extended to include new data on subcalcic, calcic, magnesian, and ferriferous variants.