A computer-implemented method of planning orthopaedic surgery comprises providing a library of templates representing orthopaedic prostheses, displaying and scaling one or more patient images such as X-ray images, allowing a user to reconfigure geometrical constructs displayed over the images to match the construct to anatomical features shown in the image; and selecting one or more templates from the library in accordance with parameters of the reconfigured constructs. The templates correspond to the orthopaedic prosthesis or prostheses which are most suitable for the patient. Hip replacement surgery can be planned using a single patient image. Knee surgery can be planned using two patient images showing different views of the anatomical features, in which case geometrical constructs for use with each view are provided. The library of templates is accessible via the Internet so as to be accessible by users in any location and readily updateable.