The study of cells, their origin, structure, function, and pathology is called cytology. Cells are continually shed (exfoliated) from tissues that line the cavities and hollow organs of the body. These exfoliated cells may float in the fluid and mucous material which bathes or passes through these cavities. These cells can be examined microscopically to determine their tissue of origin and whether or not they are malignant. The term exfoliative cytology refers to "microscopic examination of cells contained within body fluids".

The three body cavities, the pleura (enclosing the lungs), the peritoneum (enclosing the intestinal tract), and the pericardium (enclosing the heart), may be checked for fluid. The normal fluids within the body cavities are limited to an insignificant lubricating layer that cannot be aspirated. Therefore, fluid in any body cavity which can be aspirated indicates a pathological process, commonly malignant and metastatic. It is believed that the formation of malignant ascites (fluid in the abdominal cavity), for example, is brought about by colonies of cancer cells which damage the capillaries and lymphatics resulting in leakage of cancer cells and plasma directly into the abdominal cavity.

The table below lists the sources of some of the specimens that are examined histologically and some that are examined cytologically. As a further guide, histology is the study of tissues, while cytology is the study of cells.