For the non radiotherapist, the recent developments of conformal radiotherapy and Intensity Modulated Radiotherapy (IMRT) may represent merely an increase in mystification from a baseline already shrouded in the obscure dark arts of physics. However, the principles of these developments are simple and as the technologies become available, there are real opportunities to improve the local control of common cancers and to reduce some of the treatment related toxicities. Previously, planned radiotherapy involved adding the effects of a small number of co-axial radiation beams. When viewed in a cross-section these produced a distribution of isodose lines akin to the altitude lines on a contour map. Calculations were based on a cross-section through the centre of the area intended for treatment. The isodose lines were extrapolated superiorly and inferiorly to cover the length of the tumour with margins added for factors such as microscopic tumour extension and patient and internal organ motion during and between fractions. With improvements in imaging it became apparent that large volumes of adjacent normal tissues were receiving high dose radiation as a consequence of the simple field shapes. This normal tissue was at a greater risk of radiation toxicity in a volume-dependant way and limited the radiation dose safely deliverable to the target volume. Three Dimensional Conformal Radiotherapy (3D CRT) overcame this limitation with improvement in imaging and treatment planning programmes which allowed routine use of irregular beam shapes contoured to the shape of the tumour. A higher dose could potentially be delivered to the target volume