The spark of the green technology and green products has illuminated the new paths of research in the recent years. It is ecologically and economically friendly technology. This chapter provides a detailed review of the emergence of chitosan as nanoparticles. It has elaborated the history of chitosan as a vibrant biological material. It has been elaborated in detail that chitosan is a natural biodegradable and biocompatible material which has been emerged as an excellent raw material for hydrogels, tissue engineering, drug delivery applications, agriculture, and wastewater treatment. Various synthetic modes of conversion of chitin into chitosan have been described including chemicals, enzymatic, and irradiation with ultrasound radiations; each method is unique and has pros and cons. Moreover, the details of the derivative of chitosan have also been included to provide an overview about the efficacy of the chitosan for various applications. The methods for the preparation of chitosan nanoparticles have also been listed. It has been described that chitosan nanoparticles possess a number of biological properties, which have attracted considerable attention in the present scenario. The chitosan nanoparticles have been discovered as an important reinforcement in various applications, particularly biological and biomedical applications, covering a wide area of nanomedicine, tissue engineering, and drug delivery system. It has also been evaluated that the nanoparticles of chitosan are not devoid of limitations particularly associated with nanoparticles in general. It is soluble in low concentrations of inorganic acids like hydrochloric acid and acetic acid solution, which greatly affects its usage in various fields. However, this issue can be addressed by enhancing the solubility of chitosan by chemical process.