Chitin is one of the most abundant renewable resources, and chitosans, the partially deacetylated derivatives of chitin, are among the most promising functional biopolymers, with superior material properties and versatile biological functionalities. Elucidating molecular structure-function relationships and cellular modes of action of chitosans, however, it is challenging due to the micro-heterogeneity and structural complexity of polysaccharides. Lately, it has become apparent that many of the biological activities of chitosan polymers, such as in agricultural plant disease protection or in mediating scar-free wound healing, may be attributed to oligomeric break-down products generated by the action of chitosanolytic hydrolases present in the target tissues, such as human chitotriosidase. Consequently, the focus of current research is shifting toward chitosan oligomers so that the availability of well-defined chitosan oligosaccharides (COS) becomes a bottleneck. Well-known ways of producing COS use physical and/or chemical means for the partial depolymerization of chitosan polymers, typically leading to broad mixtures of COS varying in their degrees of polymerization (DP) and acetylation (DA), and with more or less random patterns of acetylation (PAs). Even after chromatographic separation according to DP and DA, such mixtures are of limited value to elucidate structure-function relationships and modes of action. More recently, enzymatic means using chitinases and/or chitosanases, and sometimes chitin deacetylases, have been proposed as these can be more tightly controlled and yield slightly better defined mixtures of COS. An alternative would be chemical synthesis of COS which in principle would allow for full structural control, but protocols for it are lengthy, costly, and not yet well developed, and yields are low. Synthetic biology now allows to develop today's in vitro bio-refinery approaches into in vivo cell factory approaches for the biotechnological production of defined COS using recombinant microbial strains expressing chitin oligomer synthases and chitin oligomer deacetylases. In this review, we will describe the state-of-the-art of this cell factory approach, as a basis for upcoming developments. We will briefly describe traditional chemical protocols and enzymatic production of COS as a background to the more detailed presentation of what has been achieved through in vivo biosynthesis. We will only briefly describe those as a background to the more detailed presentation of what has been achieved through in vivo biosynthesis. We will also touch on the production of COS derivatives that has been achieved in this way, as these oligomers open up another plethora of potential applications when used as building blocks for defined biomaterials.