Cytopathology is a branch of pathology that studies and diagnoses diseases on the cellular level. The discipline was founded by George Nicolas Papanicolaou in 1928. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to histopathology, which studies whole tissues. Cytopathology is frequently, less precisely, called "cytology", which means "the study of cells".

Georgios Nikolaou Papanikolaou was a Greek physician, zoologist and microscopist who was a pioneer in cytopathology and early cancer detection, and inventor of the "Pap smear".