Androgenetic alopecia (AGA) is the most common hair loss disorder, affecting both men and women. Due to the frequency and the often significant impairment of life perceived by the affected patients, competent advice, diagnosis and treatment is particularly important. As evidence-based guidelines on hair disorders are rare, a European consensus group was constituted to develop guidelines for the diagnostic evaluation and treatment of AGA. This S1 guideline for diagnostic evaluation of AGA in men, women and adolescents reviews the definition of AGA and presents expert opinion-based recommendations for sex-dependent steps in the diagnostic procedure.