This is a literature review examining the ability of specific MRI modalities to differentiate between benign and malignant tumours of the orbit. The authors identified 29 articles reporting diffusion weighted image (DWI) values and corresponding apparent diffusion coefficients (ADC) for biopsy proven or clinically confirmed orbital lesions. DWI is an indirect measure of the cellularity of a lesion relative to the extracellular water component of the tissue, and as cellularity increases, water diffusion becomes more restricted. However, it is affected by the T2 signal intensity, and an ADC map which removes the T2 component, compensates for this and more accurately reflects true diffusion restriction. They found primary malignant tumours had overall lower mean ADC values than benign lesions, and lymphomas had lower values than other malignant tumours. An ADC value of <0.775 x 10-3 has been suggested as a threshold to distinguished lymphomas from other orbital lesions, with a sensitivity of 96% and a specificity of 93%. An ADC of >0.92 x 10-3 was strongly suggestive of inflammatory disease rather than lymphoma, but it did not always distinguish inflammation from other non-lymphomatous lesions. Orbital metastases had a wider range of values and were harder to categorise. The authors admit that although often useful, both DWI and ADC are subject to artefact and can vary with acquisition techniques and emphasise the importance of clinical and histopathological correlation.