Standard rate equation models of island formation in the InAs/GaAs(001) system have been reassessed in terms of new experimental evidence from real time in-situ reflectance anisotropy spectroscopy (RAS) measurements. These measurements have revealed the behaviour and role of the wetting layer in the modified Stranski-Krastanov growth mode during molecular beam epitaxial growth showing that it can continue to significantly increase in thickness following the onset of islanding. The presence of two dimensional (2D) islands, which act as precursors to three dimensional (3D) islands (the quantum dots) in conventional models, does in principle allow an extension of the “wetting layer”. However, it has been found necessary to extend the standard model to include extra terms that allow material to be incorporated into (and detach from) the wetting layer and which cannot convert to 3D islands. With this improved model, it is found possible to achieve agreement with the RAS measurements.