Skeletal deformities in fish are abnormal transformations of normal bony and/or cartilaginous structures into abnormal skeletal structures that are histologically and anatomically different from their normal prototype. Such deformities are diverse in their location, morphology and impact. The three main topographic regions of a fish (head, trunk and tail) could be affected by several types of deformities with various degrees of severity depending on the cause, age and histology of the affected tissue, regardless of whether it is bony or cartilaginous. The degree of skeletal deformity could impact physiological processes, including swimming, reproduction, growth, resistance to diseases and susceptibility to predation, as well as being the direct or indirect cause of low body weight gain or even death in young fish. Skeletal deformities are commonly recorded from wild and cultured fish, with high incidence in fish hatcheries. Skeletal deformities are responsible for considerable economic damage to the sector of aquaculture by making fish unsightly and affected fish consequently remain unsold. Such deformities are also proposed as biological indicators of aquatic environmental pollution and defaults in aquaculture management. Several diagnostic techniques, such as radiography, ultrasonography and magnetic resonance imaging, have been used for detecting and defining skeletal deformities in fish. Here, the present review summarizes the incidence, classification, aetiological factors, diagnostic procedures and prevention of the most common skeletal deformities in fish.