The endoplasmic reticulum is a centrally located organelle which affects virtually every cellular function. Its unique luminal environment consists of Ca(2+) binding chaperones, which are involved in protein folding, post-translational modification, Ca(2+) storage and release, and lipid synthesis and metabolism. The environment within the lumen of the endoplasmic reticulum has profound effects on endoplasmic reticulum function and signaling, including apoptosis, stress responses, organogenesis, and transcriptional activity. Calreticulin, a major Ca(2+) binding (storage) chaperone in the endoplasmic reticulum, is a key component of the calreticulin/calnexin cycle which is responsible for the folding of newly synthesized proteins and glycoproteins and for quality control pathways in the endoplasmic reticulum. The function of calreticulin, calnexin and other endoplasmic reticulum proteins is affected by continuous fluctuations in the concentration of Ca(2+) in the endoplasmic reticulum. Thus, changes in Ca(2+) concentration may play a signaling role in the lumen of the endoplasmic reticulum as well as in the cytosol. Recent studies on calreticulin-deficient and transgenic mice have revealed that calreticulin and the endoplasmic reticulum may be upstream regulators in the Ca(2+)-dependent pathways that control cellular differentiation and/or organ development.