Bacterial fouling on surfaces is considered a major problem in modern society. Conventional methods to prevent biofilm formation often have little effect and may induce further contamination. In response to this challenge, biocidal polymers have been designed and applied as an alternative approach to kill or inhibit bacteria and prevent the formation of biofilm. These polymers can be grouped into three broad classes, namely antibiotic-releasing polymers, polymeric antibiotics, and antibiotic polymers. Antibiotic polymers are effective against bacteria both in solution and as coatings, through different mechanisms of action. In order to enhance their efficacy, biocidal polymers have been designed with single or combined biocidal and antibiofouling actions. The current review summarizes the mechanisms of action of biocidal polymers, especially antibiotic polymers, both in solution and as coatings. The review also discusses biocidal polymers with multiple functionalities.