We consider a volatile liquid droplet on a uniformly heated horizontal surface. We use lubrication theory to describe the effects of capillarity, thermocapillarity, vapor recoil, viscous spreading, contact-angle hysteresis, and mass loss on the behavior of the droplet. A new tri-junction condition, which takes into account the effect of mass loss, is derived and used. We derive an evolution equation for steady and unsteady drop profiles and solve for small and large capillary number. In the steady evaporation case, the steady contact angle is larger than the advancing contact angle. In the unsteady case, effects which tend to decrease (increase) the contact angle promote (delay) evaporation. In the large capillary number limit, we use matched asymptotics to describe the droplet profile; away from the contact line the shape is determined by initial conditions and bulk mass loss, while near the contact line surface curvature and slip are important.