This article presents an overview of nanobiosensors and nanoprobes. The fabrication, operating principle, and applications of these systems and their capacity for in vivo analysis and optical imaging at the single-cell level are discussed. Recently the cross-disciplinary integration of nanotechnology, biology, and photonics has been revolutionizing important areas in molecular biology, especially diagnostics and therapy at the molecular and cellular levels. Nanobiosensors and nanoprobes are a relatively new class of biosensing and imaging devices that allow for analytical measurements in individual living cells. These devices have the capacity to sense individual chemical species in specific locations within a cell. This article discusses the usefulness and potential of these nanotechnology-based systems in biological research and their applications to monitoring individual living cells.