BACKGROUND The lateral arm flap remains an underused flap, especially as a free flap. In this article, the authors describe the perforator anatomy to optimize flap design and harvest. METHODS Perforator locations were mapped in 12 cadavers (24 arms), and a retrospective review was conducted of 51 patients undergoing lateral arm flap surgery. RESULTS One to three reliable perforators supply the lateral arm flap. Based on cadaveric dissections, from the deltoid insertion, the A, B, and C perforators were located at 7.2 ± 1.0 cm, 9.9 ± 1.2 cm, and 11.8 ± 0.8 cm, which was 0.44, 0.61, and 0.72 of the distance from the deltoid insertion, respectively. The average pedicle length was 7.0 ± 1.1 cm. The cadavers were entirely symmetric in the number and location of the perforators between the right and left arms. All 51 patients (24 male and 27 female patients) had at least one perforator with an average pedicle length of 7.0 ± 1.3 cm, an average arterial diameter of 1.7 ± 0.3 mm, and a vein diameter of 2.5 ± 0.5 mm. All but one flap was performed as a free flap for head and neck reconstruction, with one pedicled flap for shoulder reconstruction. The average flap size was 72.2 ± 37.1 cm (range, 21 to 165 cm). The nondominant arm was used for all free flaps. There were no total or partial flap losses. Twenty-eight patients reported donor-site numbness, with one infection, one hematoma, and one wound dehiscence. CONCLUSION The lateral arm flap can be harvested reliably based on well-defined perforators and anatomical landmarks with minimal donor-site morbidity and should be included among the techniques used by reconstructive microsurgeons. CLINICAL QUESTION/LEVEL OF EVIDENCE Therapeutic, IV.