We use combined tectonic field observations and SAR data to determine an improved model of the slip associated with the 1999 Izmit earthquake, which ruptured the North Anatolian Fault at the eastern end of the Sea of Marmara. The leading goal is to understand the main features of the coseismic and post-seismic deformation, which are captured together in the SAR data. To achieve this, we make a critical analysis of the ERS1-2 SAR data, which allows atmospheric effects to be identified and removed. We also use detailed field mapping and measurements of the earthquake surface rupture. Dislocations in elastic half-space and a forward modelling strategy allow us to obtain a slip model by steps. A trial-and-error approach is combined with conventional inversion techniques to determine the slip in the different regions of the fault. The SAR data are well explained with three main zones of high slip along the fault, releasing a total moment of 2.3 x 10(20) N m (M-w = 7.6), which is higher than the seismological estimates (1.7-2.0 x 10(20) N m). The inhomogeneous slip distribution correlates with fault segments identified at the surface. The Izmit rupture appears to have extended 30 km west of the Hersek peninsula into the Sea of Marmara with slip tapering from 2 m to zero. The western end of the rupture is located 40 km SSE from Istanbul. We show that some features seen near to Mudurnu and Gevye and previously interpreted as slip on secondary faults are explained mostly as atmospheric effects correlated with the topography. Using our approach and the available GPS data we obtain a slip model that represents the coseismic slip alone, which suggests that the moment release during the main shock was 1.9 x 10(20) N m (M-w = 7.5), consistent with the seismological estimates. We conclude that the SAR data include the effects of 2 m of fast after-slip during the month following the main shock, within a zone of the fault located 1224 km below the epicentral region. Near the hypocentre at a depth of 18 km, the fault appears to have experienced dynamic slip of 1 m associated with the main shock, followed by 2 m of rapidly decelerating post-seismic shear during the following month. We suggest that the distribution of heterogeneous slip and loading along the different fault segments may be important factors controlling the propagation of large earthquake ruptures along the North Anatolian Fault.