An on tool tracking and guidance device (100, 200) for use with a surgical tool during a computer aided surgical procedure, comprising: a housing (105, 205) having a surface (120, 220) for releasable engagement with a portion of the surgical tool; a first camera and a second camera (115, 215) within the housing in an arrangement where each of the first camera and the second camera provides an image output selected for viewing a surgical field selected for a computer assisted surgery procedure using the surgical tool while the surgical tool is engaged with the housing, wherein the cameras are configured to track the relative position of the surgical tool and a patient's anatomy or marks made directly on a patient's anatomy including markers or reference frames; wherein the visual axis of the first camera and the visual axis of the second camera are inclined towards one another relative to lines generally parallel to a longitudinal axis of the housing or of the surgical tool attached to the housing, said device being characterized in that it further comprises an electronic image processor (130, 230) within the housing configured to receive an output from each of the two cameras and perform an image processing operation using at least a portion of the output from each of the two cameras for use in the computer assisted surgery procedure, the image processing operation converting the output from each of the two cameras to mathematical coordinates, relative to the surgical tool, for comparing against a surgical plan to determine if movement of the surgical tool, using the surgical tool while the surgical tool is engaged with the housing, will produce a desired resection; and a projector (125, 225) within the housing configured to provide a second output (110, 210) at least partially within the surgical field of view, wherein the second output from the projector is in a location between the first camera and the second camera, the second output from the projector comprising guidance information to guide the surgical tool, while the surgical tool is engaged with the housing, to make the desired resection based on the determination. a housing (105, 205) having a surface (120, 220) for releasable engagement with a portion of the surgical tool; a first camera and a second camera (115, 215) within the housing in an arrangement where each of the first camera and the second camera provides an image output selected for viewing a surgical field selected for a computer assisted surgery procedure using the surgical tool while the surgical tool is engaged with the housing, wherein the cameras are configured to track the relative position of the surgical tool and a patient's anatomy or marks made directly on a patient's anatomy including markers or reference frames; wherein the visual axis of the first camera and the visual axis of the second camera are inclined towards one another relative to lines generally parallel to a longitudinal axis of the housing or of the surgical tool attached to the housing, said device being characterized in that it further comprises an electronic image processor (130, 230) within the housing configured to receive an output from each of the two cameras and perform an image processing operation using at least a portion of the output from each of the two cameras for use in the computer assisted surgery procedure, the image processing operation converting the output from each of the two cameras to mathematical coordinates, relative to the surgical tool, for comparing against a surgical plan to determine if movement of the surgical tool, using the surgical tool while the surgical tool is engaged with the housing, will produce a desired resection; and a projector (125, 225) within the housing configured to provide a second output (110, 210) at least partially within the surgical field of view, wherein the second output from the projector is in a location between the first camera and the second camera, the second output from the projector comprising guidance information to guide the surgical tool, while the surgical tool is engaged with the housing, to make the desired resection based on the determination.