Recently, nanotechnology has been revolutionizing important areas in molecular biology and medicine, especially diagnostics and therapy at the molecular and cellular levels. The combination of nanotechnology, biology, advanced materials, and photonics opens up the possibility of detecting and manipulating atoms and molecules using nanodevices. This capability has the potential for a wide variety of medical uses at the cellular level. One of the most recent technological advances has been in the area of nanosensors. This chapter describes the principle of optical nanosensors, their development, and their applications for in vivo analysis of proteins and biomarkers in individual living cells. Nanosensors were fabricated with optical fibers pulled down to tips with distal ends in nanoscale dimensions. Nanosensors with immobilized bioreceptor probes (e.g., antibodies, enzyme substrate) that are selective to target analyte molecules are also referred to as nanobiosensors. Laser light is launched into the fiber, and the resulting evanescent field at the tip of the fiber is used to excite target molecules bound to the antibody molecules. A photometric detection system is used to detect the optical signal (e.g., fluorescence) originating from the analyte molecules or from the analyte-bioreceptor reaction.