There is significant potential for microfabricating tools for minimally-invasive diagnostics and therapy. Two devices were demonstrated as examples: (1) A stress sensor as a representative passive medical microsystem; (2) A cartilage mechanical property probe as an active medical microsystem. Complex bone fractures are frequently treated by using compression plates to fixate broken bones during healing. A passive ultrasonic stress sensor to be mounted on an implantable bone fixator was designed and characterized. The sensor was composed of a thin membrane mounted on a frame and subdivided into smaller micromembranes by microfabricated structures. Mathematical models were used to demonstrate load transfer between the plate and membrane, evaluate sensor design parameters, and the effect of the relative orientation of the interrogating ultrasonic transducer and the implanted sensor ond the output signal. Sensors were manufactured and evaluated in bending. The passive sensor had a linear response over the full range of forces that would be encountered in vivo. The feasibility of using an ultrasound probe to measure the dynamic response of articular cartilage arthroscopically and the microfabrication and assembly of such a device was demonstrated. Issues included alignment of the ultrasound transducer and completion of thickness measurements in the time available to a surgeon in the OR. An arthroscopic end-effector was built using the LIGA microfabrication process. Experiments were performed to assess the resolution of the device, verify that accurate thickness measurements of articular cartilage were possible, and to demonstrate measurement of the reswelling response of cartilage after indentation.