The calcium metabolism in the living body is skilfully regulated through calcium-regulating hormones, including the parathyroid hormone, active vitamin D3 metabolite and calcitonin. The blood calcium concentration is ten thousand-fold compared with the levels in cells. Cells use extracellular calcium to regulate cell function efficiently. Intracellular calcium homeostasis is exactly regulated through the mechanism related to various calcium transport systems in cells. Signal transduction plays a pivotal role in cellular regulation. Calcium signalling is probably the most ubiquitous cellular signal that mediates the action of many hormones, cytokines, and neurotransmitters. Calcium regulates the activity of numerous calcium-dependent proteins including protein kinases, protein phosphatases, protease and other calcium-regulating factors involved in calcium signalling. Calcium signalling plays a pivotal role in the regulation of manifold cellular functions including cell proliferation, apoptosis and gene expression. This book focuses on recent topics in calcium signalling and its related diseases.