A ureteral access sheath comprises a sheath assembly having a main lumen and one or more secondary lumens. The sheath assembly can be configured with medical devices in both lumens, for example, a ureteroscope in the main working channel, and a guidewire, stone basket, grasper, laser fiber, or other surgical instrument in the secondary working channel. Or the sheath assembly can be configured with a medical device in one channel and the other lumen coupled to an irrigation means, such that irrigation of the surgical field can be efficiently accomplished even though the main working channel is substantially completely occupied by, e.g., a ureteroscope. Or the sheath assembly can be configured for irrigation through one lumen and aspiration through the other lumen, thereby creating a turbulent flow which washes the surgical field to facilitate removal of particles and debris.