A customized patient-specific orthopaedic instrument, comprising a customized patient-specific femoral cutting block that comprises: (i) an anterior body having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of an anterior side of a patient's femur that has a corresponding positive contour, the body providing a cutting slot for guiding the blade of a saw, the cutting slot being formed in the anterior body or in a cutting guide which is coupled to the anterior body and formed from a material different from the body, the cutting slot being positioned to allow a surgeon to perform a distal cut on the patient's femur, (ii) a first tab extending posteriorly from the body, the first tab having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the distal end of the patient's femur that has a corresponding positive contour, (iii) a first lip extending superiorly from the end of the first tab, the first lip having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the posterior side of the patient's femur that has a corresponding positive contour, characterised in that the instrument includes: (iv) a second tab extending posteriorly from the body, the second tab having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the distal end of the patient's femur that has a corresponding positive contour, so that the first and second tabs define an opening between them, (v) a second lip extending superiorly from the end of the second tab, the lip having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the posterior side of the patient's femur that has a corresponding positive contour, in which each of the first and second lips (1408, 1410) has a blind groove extending laterally across its bone-facing side which is in a common transverse plane with the cutting slot provided by the anterior body, so that the grooves can receive the distal end of a saw blade which is inserted through the cutting slot provided by the anterior body. (i) an anterior body having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of an anterior side of a patient's femur that has a corresponding positive contour, the body providing a cutting slot for guiding the blade of a saw, the cutting slot being formed in the anterior body or in a cutting guide which is coupled to the anterior body and formed from a material different from the body, the cutting slot being positioned to allow a surgeon to perform a distal cut on the patient's femur, (ii) a first tab extending posteriorly from the body, the first tab having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the distal end of the patient's femur that has a corresponding positive contour, (iii) a first lip extending superiorly from the end of the first tab, the first lip having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the posterior side of the patient's femur that has a corresponding positive contour, characterised in that the instrument includes: (iv) a second tab extending posteriorly from the body, the second tab having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the distal end of the patient's femur that has a corresponding positive contour, so that the first and second tabs define an opening between them, (v) a second lip extending superiorly from the end of the second tab, the lip having a bone-facing surface having a customized patient-specific negative contour configured to receive a portion of the posterior side of the patient's femur that has a corresponding positive contour, in which each of the first and second lips (1408, 1410) has a blind groove extending laterally across its bone-facing side which is in a common transverse plane with the cutting slot provided by the anterior body, so that the grooves can receive the distal end of a saw blade which is inserted through the cutting slot provided by the anterior body.