Understanding the wetting and evaporation behaviour of volatile droplets on heated surfaces is very important for many industrial applications. In this paper the behaviour of a sessile drop evaporating on a heated surface is investigated both experimentally and numerically. Results are reported for the evaporation of water drops on two different substrates at various temperatures. A numerical model, based on a finite element method, has been developed to describe the hydrodynamics inside the evaporating drop and the effect of the humidity on the evaporation process, assuming the droplet to be a spherical cap. The energy and Navier-Stokes equations are solved within the droplet and the vapour concentration is computed using the diffusion equation. The drop volume and flow and temperature fields within the drop are obtained and the evolution of the volume in time is compared with the experimental results.