Nanocellulose-reinforced polymeric composites can be produced in situ, where polymerization occurs in the presence of reinforcing fillers. Moreover, it can also be produced ex situ, when, the polymeric matrix is mixed with a nanocellulose suspension. This paper provides a review covering technical information, including physical, chemical, and procedural characteristics of the polymerization process. It explores the use of three different in situ techniques: suspension, semi-suspension, and emulsion in the presence of nanocellulose suspensions to produce nanocellulose-reinforced composites. The polymerization techniques addressed in this work, when in the presence of nanocellulose, generally yield biodegradable materials from a renewable source and with wide applicability.