The book is a compendium of my doctoral research, through which I attempted to address one of the most fundamental questions, “how do the nanocrystals grow?” The parameters affecting the particle size, monodispersity and luminescence of semiconductor nanocrystals were identified. Supersaturation was established as the chief architect of nanoparticle characteristics. The particles were surface modified and the interaction of these highly luminescent, biofunctionalized quantum dots (QDs) with some biomolecules was studied. From these works, new prototcols have emerged for the QD-based biosensors, eg. fluorimetric detection of tyrosine at nanomolar level using CdS QDs. A novel CdTe QD-based method for enzyme assay was also developed. At the nano-bio interface, our interest was aroused by an inorganic molecule peroxynitrite (ONOO-), which has been implicated as a causative agent in a number of neuro-degenerative and inflammatory diseases. The volatile interface between this reactive oxygen species (ROS) and bio-functionalized CdS and CdTe quantum dots was investigated. It provided some new insights into the ROS reactivity and QD-luminescence thus advancing the nanodiagnostics.