A system for controlling the trajectory of a surgical instrument, comprising: an orientation sensor; a surgical instrument (12, 80) having a proximal portion, a distal portion, and a longitudinal axis extending between said proximal portion and said distal portion, said distal portion configured for advancement through body tissue to a target surgical site, and said proximal portion configured to remain outside said body tissue when said distal portion is advanced to said target surgical site and to receive said orientation sensor removably coupled thereto such that said surgical instrument is orientable along a desired trajectory; said orientation sensor operable to determine a first angular relationship in a first plane between said orientation sensor and a reference direction and operable to determine a second angular relationship in a second plane between said orientation sensor and said reference direction, wherein said second plane is orthogonal to said first plane and said orientation sensor is removably coupled to said surgical instrument (12, 80), wherein said orientation sensor is operable while said distal portion of said surgical instrument (12, 80) is positioned within said body tissue; and a feedback device communicatively coupled to said orientation sensor and configured to communicate information to a user regarding at least one of said determined first and second angular relationships between said orientation sensor and said reference direction; wherein said surgical instrument (12, 80) is further configured to angulate about said distal portion until said feedback device indicates that the angulation orientation of said surgical instrument longitudinal axis in said first plane matches the desired instrument trajectory in said first plane and the angular orientation of said surgical instrument longitudinal axis in said second plane matches the desired instrument trajectory in said second plane, wherein said orientation sensor comprises at least one accelerometer, wherein the system is a spinal surgery system for controlling the trajectory of a surgical instrument in spinal surgery; wherein the surgical instrument is at least one of a pedicle hole formation instrument or a pedicle screw insertion instrument; wherein said reference direction is gravity; characterised in that: said orientation sensor is removably coupleable to said surgical instrument only in a single known orientation. an orientation sensor; a surgical instrument (12, 80) having a proximal portion, a distal portion, and a longitudinal axis extending between said proximal portion and said distal portion, said distal portion configured for advancement through body tissue to a target surgical site, and said proximal portion configured to remain outside said body tissue when said distal portion is advanced to said target surgical site and to receive said orientation sensor removably coupled thereto such that said surgical instrument is orientable along a desired trajectory; said orientation sensor operable to determine a first angular relationship in a first plane between said orientation sensor and a reference direction and operable to determine a second angular relationship in a second plane between said orientation sensor and said reference direction, wherein said second plane is orthogonal to said first plane and said orientation sensor is removably coupled to said surgical instrument (12, 80), wherein said orientation sensor is operable while said distal portion of said surgical instrument (12, 80) is positioned within said body tissue; and a feedback device communicatively coupled to said orientation sensor and configured to communicate information to a user regarding at least one of said determined first and second angular relationships between said orientation sensor and said reference direction; wherein said surgical instrument (12, 80) is further configured to angulate about said distal portion until said feedback device indicates that the angulation orientation of said surgical instrument longitudinal axis in said first plane matches the desired instrument trajectory in said first plane and the angular orientation of said surgical instrument longitudinal axis in said second plane matches the desired instrument trajectory in said second plane, wherein said orientation sensor comprises at least one accelerometer, wherein the system is a spinal surgery system for controlling the trajectory of a surgical instrument in spinal surgery; wherein the surgical instrument is at least one of a pedicle hole formation instrument or a pedicle screw insertion instrument; wherein said reference direction is gravity; characterised in that: said orientation sensor is removably coupleable to said surgical instrument only in a single known orientation.