A time-resolved volumetric 3D particle tracking tool is presented for mixing in stirred tanks studies and its capabilities are explored in a low Reynolds number custom-made stirred tank. The technique provides a Lagrangian description of particles in the flow resolving their 3D trajectories, velocities, and acceleration. The optimization of the system allowed, for the first time, tracking similar to 400 particles at a time in an imaged volume (60 mm x 60 mm x 50 mm) in the stirred tank. The system was validated in a 1-D piston-driven flow and by comparing the measurements with a 2D-particle image velocimetry system in a stirred tank. Results show the advantages of this technique to understand particle dynamics. Sample measurements illustrated that small changes in the stirrer configuration produced an unexpected variety of particle trajectories which were classified according to their path shape and location. Particle velocities and Lagrangian accelerations provided new insights into their mixing process. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1983-1996, 2011