Multifactorial conditions are common health problems and rare conditions caused by a combination of both genetic and other factors that may include internal factors such as ageing and exposure to external environmental factors such as diet, lifestyle, and exposure to chemicals or other toxins.

Multifactorial conditions have in common that they do not always develop despite the suggested presence of an affected gene(s). The inherited affected gene(s) make the person at increased risk for developing the condition (predisposed or susceptible) but unless other factors are present, the condition may never develop at all.