Some of the basic aspects of the scrape-off layer, SOL, of magnetically confined plasmas are reviewed. SOLs defined by limiters and divertors are compared. The role of parallel heat convection versus conduction is considered and the 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 Langmuir probes built into divertor targets or limiters. The model is applied to a DIII-D discharge and comparisons are made between the model and measurements made by Thomson scattering and photodiodes. Values of D⊥SOL and χ⊥SOL are extracted from the experimental data using the simple model.