P16 immunohistochemical expression, a surrogate marker of the retinoblastoma pathway, has become a major adjunct in the routine practice mostly of cervical and head/neck pathology, but with other indications too. In this chapter, a detailed immunohistochemical technique for the detection of p16 is described, followed by indications and interpretation of its expression in uterine, ovarian, vulvar, penile, head-and-neck, melanocytic, and other pathologies.