Worldwide pollution of waters and soils is a rising health issue calling for advanced decontamination methods such as adsorption of contaminants on biochar because biochar is carbon-negative material displaying many adsorptive properties. Biochar has been improved by chemical, physical and magnetic modifications, and by surface decoration, yet the recent development of biochar-supported composites appears more promising. Here we review biochar-supported composites with focus on preparation, mechanisms, biochar-clays, biochar-microorganisms, biochar-enzymes, and applications to adsorb metals, nutrients and organic contaminants.