A pilus (Latin for 'hair'; plural: pili) is a hair-like appendage found on the surface of many bacteria and archaea. The terms pilus and fimbria (Latin for 'fringe'; plural: fimbriae) can be used interchangeably, although some researchers reserve the term pilus for the appendage required for bacterial conjugation. All conjugative pili are primarily composed of pilin – fibrous proteins, which are oligomeric.

Dozens of these structures can exist on the bacterial and archaeal surface. Some bacteria, viruses or bacteriophages attach to receptors on pili at the start of their reproductive cycle.

Pili are antigenic. They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity. Specific host responses to old pili structures are not effective on the new structure. Recombination betwene genes of some (but not all) pili code for variable (V) and constant (C) regions of the pili (similar to immunoglobulin diversity). As the primary antigenic determinants, virulence factors and impunity factors on the cell surface of a number of species of Gram negative and some Gram positive bacteria, including Enterobacteriaceae, Pseudomonadaceae, and Neisseriaceae, there has been much interest in the study of pili as organelle of adhesion and as vaccine components. The first detailed study of pili was done by Brinton and co-workers who demonstrated the existence of two distinct phases within one bacterial strain: pileated (p+) and non-pileated)