An alignment trial system for a hip prosthesis comprising: (a) a trial femoral prosthesis comprising a head component, a neck component and a stem component; and (b) a trial acetabular cup prosthesis wherein the head component of the trial femoral prosthesis and the trial acetabular cup prosthesis include interlocking engagement formations and wherein the trial acetabular cup prosthesis includes means to facilitate the formation of a bore in the pelvis. The invention also relates to a method of inserting an acetabular cup prosthesis comprising the steps of: (I) preparing the femur and pelvis for insertion of a femoral prosthesis and an acetabular cup prosthesis; (ii) inserting the alignment trial apparatus of the above first aspect such that the trial femoral prosthesis is located in the prepared femur; (iii) locating the patient's hip such that the trial cup prosthesis is seated in the prepared portion of the pelvis; (v) drilling a bore in the pelvis; (vi) removing the alignment trial apparatus; (vii)inserting the acetabular cup prosthesis into the pelvis using the mark as a guide.