This review describes the recent advances in production of cellulose nanofibrils (CNF), otherwise known as nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC) or cellulose nanofibers, a material with significant barrier, mechanical and colloidal properties, low density, renewable and biodegradable character. The above properties make CNF promising for applications in such fields as papermaking, composites, packaging, coatings, biomedicine and automotive. In this review, CNF production methods are summarized, covering raw materials selection, structural and chemical aspects necessary for understanding the nanofibril extraction process, conventional and novel mechanical disintegration techniques, as well as biological and chemical pretreatments aimed at facilitating nanofibril isolation. The issues of fractionation, performed with the objective of retrieving the residual microscopic fiber fragments from CNF suspensions, are addressed. Additionally, the preparation of CNF in various forms, such as suspensions, water-redispersible powders, films or nanopapers, hydrogels and aerogels, is discussed.