钙对生物体而言是必需的营养物质，在绝大多数的生物体内皆具有显著影响生理功能与生物化学功能不可或缺、且极为重要的角色。。 本页面是用以讨论钙质在生物体中的相关用途与机制。

生物必须借由来自外在环境的供应来补充每日固定的流失，这包含但不只限于摄食，或是单指细胞必须透过许多机制将钙从环境透过信道蛋白进入细胞中。

在动物的营养学的相关领域里，中文对矿物质的称呼习惯为「元素名称后面加上一个"质"字」。以钙元素为例，"钙"通常会被称呼为「钙质」并将「钙质」一词用以形容在动物体内的钙(包含但不限于以离子形式呈现的元素态，或是与螯合蛋白组合成的复合物形式。 )，而较少仅单以「钙」一词作为称呼。

Calcium ions (Ca) play a vital role in the physiology and biochemistry of organisms and the cell. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization. Many enzymes require calcium ions as a cofactor, those of the blood-clotting cascade being notable examples. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.