The Johnson and Ettinger model was developed in the USA to estimate vapour migration into build-ings. It has been incorporated into many software tools that are commonly used in the UK for contaminated-land risk assessment (e.g. CLEA, RISC, RBCA). It is commonly accepted that the model over-estimates vapour migration into buildings by at least an order of magnitude. Many practitioners use the model and software without fully understanding how the equations were derived and how the ‘black box’ is modelling the flow of ground gas or vapours into buildings. This paper will discuss how the Johnson and Ettinger approach models vapour migration into buildings, and will highlight key factors that may make the use of the model invalid for many of the new buildings being constructed in the UK. An alternative simple modular approach is proposed that uses common equations to model phase partitioning, vapour transport and dilution mechanisms. The equations used to define these mechanisms are derived from the same basic theory as those used in the Johnson and Ettinger model or other methods such as the VOLASOIL models. They are applied in a way that is arguably more flex-ible and transparent. The approach can also be used to model the effect of introducing protection measures to reduce vapour migration into a building.