消化作用是指将食物分解成足够小的分子使身体能够吸收利用的过程，是生物异化作用（分解代谢）的一环。

大多数食物中所含的有机物包括蛋白质、脂肪和碳水化合物。由于这些大分子聚合物无法穿过细胞膜进入细胞内，而且动物需要用单体来合成自身身体所需的聚合物，因此动物需要借由消化作用将食物中的大分子分解成单体。例如将蛋白质分解为胺基酸，多糖及双糖分解为单糖，脂肪分解为甘油及脂肪酸等。消化作用分为两个阶段，首先借由机械性的作用将食物碎裂成小裂片，其次是化学性的作用，经由酵素的催化，将大分子水解成小分子单体。而无法消化的残渣则会再排出体外。

Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the body can use.