In this educational review the relative merits and current status of computed tomography (CT), magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) in the planning of radiotherapy are discussed. Following a general introduction to the requirements of medical imaging in this context each imaging modality is described in turn. Computed tomography is by far the most widely used, being fast, geometrically precise and providing the electron density information required for dose calculations. Historically MRI has been little used owing to the presence of geometric distortions and lack of electron density information. However the superior tissue contrast to CT has stimulated strategies to overcome these limitations. Co-registration to CT images solves both problems but is non-trivial and requires both sets of exams to be performed. Alternatively methods are available to correct distortions and to segment images to make good estimates of electron density. Magnetic Resonance Spectroscopy probes tissue biochemistry, showing good contrast between tissues, and showing treatment response sooner than other modalities. However spatial resolution is poor (∼ 1cm) and the method can be slow. Spectra are intrinsically co-registered to MR images acquired in the same examination, but require the same corrections for geometry and electron density information as MRI.