Diffuse androgenetic alopecia (female pattern hair loss), telogen effluvium, and diffuse alopecia areata may have similar clinical manifestations. Subtle details on physical examination and dermoscopy of the scalp may help to identify those disorders. The authors present a practical discussion on how to approach the patient with diffuse alopecia, considering clinical history, physical examination, and dermoscopic findings. If the diagnosis remains unclear after a careful analysis of the clinical signs, a scalp biopsy may help to distinguish between the three diseases. In this first part of our study, an objective review of female androgenetic alopecia is presented and the most important histological changes are discussed.