Epithelial cells probably constitute the most diverse group of cells found in the body. In addition to serving as interfaces between external and internal environments, their functions include ion and fluid secretion and reabsorp­ tion, protein exocytosis, hormone secretion, recognition, surface protection and the control of ciliary movement. By their very exposure on the surfaces of the body, epithelial cells are subjected to wide-ranging assault, by micro­ organisms and by chemical and physical forces. They are the targets for abrasion, infection and malignant transformation. Some epithelial cells show altered behaviour in inherited syndromes, such as cystic fibrosis, characterized by serious pancreatic and pulmonary disease. In view of the importance of epithelia and the fact that their function can be altered by environmental and inherited factors, they are the subject of intensive research, particularly so in the case of cancer where most tumours are of epithelial origin. The use of animal tissues in epithelial research continues to provide important advances and this, coupled with the need to focus more on human tissues, has prompted a greater research emphasis on accessible human epithelia and on the establishment of cell cultures from animal and human sources. For primary cell cultures and cell lines to be of value, they need to express properties appropriate to their progenitors and relevant to the study in progress.