We demonstrate the flexibility of a multiple-beam trapping system that enables interactive manipulation of fluid-borne colloidal structures with advanced controllability and versatility that can lead to light-powered microdevices performing multiple functions in a "lab-on-a-chip". A straightforward phase-imaging operation forms the basis for the efficient generation of arbitrarily shaped trapping beam configurations. The multiple-beam trapping pattern is a direct map of the phase variation encoded on a programmable phase-only spatial light modulator (SLM). A graphical user interface that encodes desired phase patterns onto the SLM enables interactive and independent control over the dynamics and geometry of each trapping beam. Experimental results show that the system can be used for guided assembly of particles in a plane, control of particle stacking along a beam axis, and real-time sorting of inhomogeneous mixtures of microspheres. These experiments illustrate that multiple beams generated by the system can be utilized not only for the improved synthesis of functional microstructures but also for their non-contact and parallel actuation crucial for sophisticated microfluidic-based lab-on-a-chip demonstrations in the future.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.