A hip resurfacing CAS system for guiding an operator in resurfacing a femoral head in computer-assisted surgery for subsequent implanting of a femoral head implant, comprising: a trackable reference on the femur, the trackable reference being trackable to form a femoral frame of reference of the femur; a registration tool being trackable; at least one bone-altering tool associated with a resurfacing of the femoral head, the at least one bone-altering tool being trackable; a tracking apparatus for tracking the trackable reference, the registration tool and the at least one bone-altering tool; and a resurfacing processing unit connected to the tracking apparatus so as to receive tracking data for the trackable reference, the registration tool and the at least one bone-altering tool, the resurfacing processing unit having: a position/orientation calculator to calculate from the tracking data a position and/or orientation of the trackable reference to track the femoral frame of reference, and of the registration tool and the at least one bone-altering tool; a model generator receiving position and/or orientation data of the registration tool to produce a model of the femoral head and neck with respect to the femoral frame of reference; and a resurfacing evaluator determining an evaluated bone resurfacing alteration of the femoral head as a function of a position and/or orientation of the at least one bone-altering tool with respect to the bone model of the femoral head and neck, and a tool geometry model, at least prior to resurfacing being performed. a trackable reference on the femur, the trackable reference being trackable to form a femoral frame of reference of the femur; a registration tool being trackable; at least one bone-altering tool associated with a resurfacing of the femoral head, the at least one bone-altering tool being trackable; a tracking apparatus for tracking the trackable reference, the registration tool and the at least one bone-altering tool; and a resurfacing processing unit connected to the tracking apparatus so as to receive tracking data for the trackable reference, the registration tool and the at least one bone-altering tool, the resurfacing processing unit having: a position/orientation calculator to calculate from the tracking data a position and/or orientation of the trackable reference to track the femoral frame of reference, and of the registration tool and the at least one bone-altering tool; a model generator receiving position and/or orientation data of the registration tool to produce a model of the femoral head and neck with respect to the femoral frame of reference; and a resurfacing evaluator determining an evaluated bone resurfacing alteration of the femoral head as a function of a position and/or orientation of the at least one bone-altering tool with respect to the bone model of the femoral head and neck, and a tool geometry model, at least prior to resurfacing being performed. a position/orientation calculator to calculate from the tracking data a position and/or orientation of the trackable reference to track the femoral frame of reference, and of the registration tool and the at least one bone-altering tool; a model generator receiving position and/or orientation data of the registration tool to produce a model of the femoral head and neck with respect to the femoral frame of reference; and a resurfacing evaluator determining an evaluated bone resurfacing alteration of the femoral head as a function of a position and/or orientation of the at least one bone-altering tool with respect to the bone model of the femoral head and neck, and a tool geometry model, at least prior to resurfacing being performed.