This paper was presented at the Twentieth National Conference of the Society of Aeronautical Weight Engineers at Akron, Ohio, May 15 – 18, 1961. This paper discusses the approach and problems encountered during the phases that led to the successful completion of an operative trifilar moment of inertia test apparatus. The investigation of three types of pendulums--- torsional, compound, and bifilar--- is discussed and the final selection of a trifilar pendulum is explained. The use of the trifilar allows the weight, centers of gravity, and moments of inertia about one axis to be obtained with one setup, thereby reducing handling. The paper discusses the methods used in the fabrication and assembly of the test equipment as well as its calibration and the calculation of tare values. The paper concludes with the description of operational procedures for performing the desired measurements.