Squamous cell carcinoma (SCC) of the lower lip is a deadly nonmelanoma skin cancer. Its precursor, a distinctive cutaneous neoplasia analogous to cervical dysplasia, is known by the confusing term actinic cheilitis. Solar cheilosis (SC) is a more appropriate designation. It represents incipient SCC in situ. SC is widely recognized as an ultraviolet light–induced precancer of the lower lip that is typically seen in light-skinned individuals and others with poorly pigmented lower lips. Lip SCC is one of the most common malignancies of the oral cavity. SCC is much more likely to metastasize from the lip than cutaneous surfaces, with a 5-year overall survival rate of less than 75%. SC results from long-term exposure to ultraviolet radiation. The occurrence of SC is dose-dependent and is influenced by the patient’s solar exposure, age, genetic predisposition, geographic latitude of residence, occupation, leisure activities, and use of lip protective agents. Molecular abnormalities of SC are similar to those of actinic keratosis and facilitate the evolution to SCC. A high degree of clinical suspicion should be maintained, given the malignant nature of this condition. Ulceration and nodularity often indicate progression to SCC. We performed a Medline and Google Scholar search for all articles related to actinic cheilitis, actinic cheilosis, SC, actinic keratosis, solar keratosis, premalignant oral disease, and lip SCC, and have also evaluated many other articles and book chapters. One hundred forty-two peer-reviewed articles were identified as being of particular value. Pertinent facts were selected and analyzed.