This paper presents an explanation for the logarithmic spiral plan form commonly observed in headland-bay beaches. The analysis focuses on the equilibrium state of such beaches. Basic ideas on longshore currents and wave refraction and diffraction are cast into simple analytical expressions and combined with an empirical relation between wave energy, grain size and beach slope to construct synthetic beach forms. The plan shapes predicted by theory are closely fitted by the logarithmic spiral and compare well with natural beaches. The model shows the interplay of the various factors responsible for the observed beach shape and provides a simple framework for the interpretation of a common feature of many shorelines.