Chitosan is the deacetylated form of chitin, a biopolymer that can be obtained from various waste streams and shows antimicrobial activity against different fungi, gram positive and gram negative bacteria. Although chitosan has been studied for years, many aspects remain unclear. This review gives an overview of the antimicrobial activity of chitosan and novel chitosan derivatives of the last 15 years. The most important parameters (molecular weight, degree of deacetylation, etc.) are discussed along with a status update on the mode of action of chitosan.