Calreticulin (CRT) is an endoplasmic reticulum luminal calcium-binding protein with multiple cellular functions, including intracellular Ca2+ homeostasis, oxidative stress responses, and lectin binding. CRT can also modulate cell adhesion, cell-cell interactions, migration, phagocytosis, integrin-dependent Ca2+ signaling, and immune responses, and plays an important role in cellular proliferation, differentiation, and apoptosis. Given these roles, it is not surprising that CRT function has important implications in health and disease. Considerable evidence in recent years suggests that CRT dysfunction is associated with cancer and that CRT could be a diagnostic marker and a target for cancer therapy. These topics are discussed in depth in this review.