The lymphatic system acts to return lower-pressured interstitial fluid to the higher-pressured veins by a complex network of vessels spanning more than three orders of magnitude in size. Lymphatic vessels are comprised of lymphangions, segments of vessels between two unidirectional valves, which contain smooth muscle that cyclically pump lymph against a pressure gradient. Whereas the principles governing the optimal structure of arterial networks have been identified by variations of Murray's Law, the principles governing the optimal structure of the lymphatic system have yet to be elucidated, although lymph flow can be identified as a critical parameter. The reason for this deficiency can be identified. Until recently, there has been no algebraic formula like Poiseuille's Law which relates lymphangion structure to its function. We therefore employed a recently-developed mathematical model, based on the time-varying elastance model conventionally used to describe ventricular function, which was validated by data collected from post-nodal bovine mesenteric lymphangions. From this lymphangion model, we developed a model to determine the structure of a lymphatic network that optimizes lymph flow. The model predicted that there is a lymphangion length that optimizes lymph flow, and that symmetrical networks optimize lymph flow when the lymphangions downstream of a bifurcation are 1.26 times the length of the lymphangions immediately upstream. Measured lymphangion lengths (1.14 ± 0.5 cm, n=74) were consistent with the range of predicted optimal lengths (0.1 to 2.1 cm). This modeling approach was possible because it allowed a structural parameter such as length to be treated as a variable.