Radiotherapy usually involves directing high-energy X-rays from a linear accelerator to kill cancer cells, but as radiation will damage normal as well as cancerous cells, beams must be accurately delivered to keep side effects to a minimum. Radiotherapy Physics as a specialism employs the highest number of Clinical Scientists in Medical Physics, although they are largely unnoticed by patients as direct patient contact is rare. Radiobiology can inform radiotherapy practice at various levels. On a conceptual basis, modelling mechanisms and processes that explain observed responses of both tumour and normal tissues can be used to propose developments and adjustments to treatment strategies. Radiation damage and cancer cell death, while minimising damage to healthy cells, is the main purpose of radiotherapy treatment. The therapeutic index is a concept that relates the dose response for tumour control to the dose response curve for the normal tissue complications.