Gravity datasets from two geothermal areas in Iceland, an Iceland-wide gravity dataset, and marine gravity data from the Reykjanes Ridge at 58˚N, which can be seen as the submarine extension of the Icelandic accretionary system, have been studied in this thesis. Results of previous local earthquake seismic tomography studies of the shallow crustal structure of two spreading segments in Iceland have been used, in conjunction with gravity observations, to assess the fidelity of the tomographic method and further refine the description of the crustal structure for these areas. This was accomplished by predicting the component of the observed gravity field which may be produced by the tomographically imaged bodies and in the process attempting to explain any residual anomalies. Results showed additional shallow structure inferred from gravity observation and suggested that local earthquake seismic tomography can 'overlook' some geological structure. The Icelandic gravity field was separated into long and short wavelength components which were attributed to mantle plume and crustal effects, respectively. Physical parameters were attributed to the sources of these anomalies where possible. Results showed that a simple cylinder model (radius -100 km) of anomalous mantle density (~ - 35 kg m(^-3)) could explain the large scale gravity field over Iceland. Shallow density variations in the top 1 km of the crust appeared to be mainly responsible for smaller scale gravity anomalies. A simple Bouguer slab model suggests that the crust may be ~7 km thicker beneath Iceland compared to neighbouring oceanic areas, consistent with an underplating mechanism for crustal accretion in Iceland. Gravity data were acquired on a cruise over the Reykjanes Ridge in the North Atlantic. The marine gravity data were reduced systematically to a residual anomaly and showed that there was the possibility of crustal thinning associated with a bathymetric offset which was interpreted as a second order discontinuity. The form of the residual gravity was similar to other discontinuities of this order on the Mid-Atlantic Ridge, although not of the same amplitude.