The North Atlantic and North Pacific commercial fisheries report problematic occurrences of ‘jellied’ or ‘mushy’ fish. These fish exhibit an abnormally soft and jelly-like musculature that attains a mushy consistency when cooked. The condition affects several economically important species, and is commonly termed ‘jellied condition’ or, specifically for halibut, ‘mushy halibut syndrome’. The inferior quality of ‘jellied’ or 'mushy' products reduces the market value considerably, leading to wastage and losses in the fisheries. The syndrome is associated with an abnormally high moisture content and lowered protein of the white skeletal musculature, particularly the fillets. Alterations in lipid content varies depending on species investigated. In some fish species, myxozoan infections can induce similar fillet alterations, but studies on ‘jellied’ or ‘mushy’ meat indicate a non-infectious myopathy. Several hypotheses have been launched to explain the jelly-like syndrome, including dietary deficiencies, spawning exhaustion, environmental circumstances, genetics and adaptive physiology. This review provides a comprehensive overview of the characteristics of the ‘jellied’ or ‘mushy’ syndrome including a discussion of plausible etiologies and applicable mitigation strategies. The main conclusion is that the syndrome may involve two different etiologies dependent on species and location, but new studies are needed to verify past findings and rule out alternative explanations. A growing human population and an increasing demand for food requires efficient utilization of all harvested fish products. Expanded and updated knowledge is vital to reduce food waste and losses related to ’jellied’ or ‘mushy’ fish in catches, and promote sustainable quota usage. We advocate for further research into the syndrome, including prevalence in commercial fish stocks and correlational studies with focus on association with biological parameters, chemical composition, infectious agents, geographic distribution and seasonal variation.