A fast moving research area where there is an ever-expanding interest, and which impacts upon a wide variety of genetic diseases. Important introductory section which considers the merits of methods employed in these studies, and examines factors which influence study design. Looks at the different approaches that have been taken to study complex inherited diseases by considering a variety of common diseases as models. Discusses the successes achieved through past studies, with implications for future research. Multifactorial or complex diseases are those characterized by increased risks within families, caused by more than one gene, and which predominantly have a tremendous impact on morbidity and mortality in the general public. Examples of multifactorial diseases include: common cancers - breast, bowel, ovary etc.: Alzheimer's; epilepsy; diabetes; multiple sclerosis; schizophrenia and manic depression; asthma; rheumatoid arthritis etc.