Cellulose is one of the most abundant renewable natural resources. With the development of nanotechnology, cellulose scientists' focus has shifted to the extraction of nanocellulose from various plant sources and its use in technical applications. The promising features of nanocellulose and its products are the driving force behind the rapid progress in nanocellulose chemistry and engineering. In fact, nanocelluloses combine the important characteristics of cellulose with the extraordinary features of nanoscale materials. This paper summarizes the current knowledge in the research and development of nanocelluloses. Nanocellulose, or variously termed nanocrystals, whiskers, rods, nanofibrils, or nanofibers, is when the cellulose fiber or crystal has at least one dimension within the nanometer size range. After an introduction to the chemical constitution and microfibrillar arrangement of nanocellulose in the cellulose bundles together with other constituents, the ways of synthesizing nanocellulose and its functionalization approaches will be discussed. Finally, the use of nanocellulose for the detection of pollutants will be introduced.