Many ecologists and aquaculturists view crayfish as either detritivores or herbivores. The nutritional importance of detritus and plant material, seemingly confirmed by examination of stomach contents is exaggerated. Examination of stomach contents emphasizes indigestible residues, for example, plant fragments and detritus, while neglecting the ingestion of easily digestible soft‐bodied metazoans. The few comparative studies that have measured the assimilation and metabolic use of ingested items reveal animal protein to be the chief substance promoting rapid growth. Crayfish are not indiscriminate omnivores; in fact, they demonstrate a predilection for animal protein. Crayfish rank among the chief carnivores found in lakes and streams. To completely understand their role, crayfish should be viewed from two perspectives: that of an individual attempting to complete its life cycle, and the collective functioning of a population in the ecosystem, which can produce a serious ecological impact. While searching for and ingesting animal protein, they can ingest large amounts of herbaceous and detrital materials. As sources of animal protein are exhausted, crayfish become facultative herbivores. After reaching high densities, their extended foraging activity can modify habitat, for example, destruction of macrophyte beds, algal mats, etc. In an extreme situation, they can seriously reduce or even eliminate other animals and plants from the ecosystem. Hence, crayfish often act as keystone predators and serve as major conduits of energy and materials in complex food webs.