Cell-based tissue engineering efforts, involving the incorporation of primitive (e.g., undifferentiated, pluripotent, stem) cells into biomaterial scaffolds, represent a significant research thrust in the field of regenerative medicine [7, 55, 65, 77]. The ability for these engineered tissues to regenerate functional tissues or organs hinges on the ability of the cellular component to differentiate, organize into tissue-like structures, and provide physiological signals to guide integration, vascularization, and normal remodeling processes in vivo. Despite the promise of primitive cells in tissue-regeneration strategies, they are highly complex and dynamic, and their behavior – even in well-controlled studies – is difficult to predict. Consequently, their use demands extensive characterization of both cell/biomaterial interactions in vitro, and engineered tissue/host interactions in vivo. For example, cells may be seeded into biomaterial matrices and allowed to proliferate, migrate, and mature (i.e., to differentiate and acquire specialized functions) over many days in a bioreactor. During this culture period, the ability to monitor and evaluate changes in phenotype and function, noninvasively, in detail, and in real-time could translate into significant improvements in the engineered tissue products.