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A simple and general method of optical rotation of microparticles in optical traps is demonstrated, avoiding all of the problems associated with the usual methods of rotation. The applied torque are optically measured; making the technique useful for a wide range of quantitative applications using naturally occurring particles as well as for orientational manipulation of specimens. Generation of optical torque is modelled by calculating the scattering by the particle in the trap using the T-matrix method. Results confirm that the torque results from the transfer of spin angular momentum.