Some of the basic aspects of the scrape-off layer, SOL, of magneticallyconfined plasmas are reviewed. SOLs defined by limiters and divertors arecompared. The role of parallel heat convection versus conduction is considered andthe relation to the desired condition of large ∇∥T along the SOL is assessed.A very simple SOL model is developed to calculate the two-dimensional `fields' in the SOL,i.e. T(s∥,s⊥),n(s∥,s⊥), using measured boundary conditions from Langmuirprobes built into divertor targets or limiters. The model is applied to aDIII-D discharge and comparisons are made between the model and measurementsmade by Thomson scattering and photodiodes. Values of D⊥SOLand χ⊥SOL are extracted fromthe experimental data using the simple model.