•Biochar as a bulking agent possesses constructive features to assist composting.•Effect of biochar on composting depends on biochar and feedstock properties.•Biochar decreases bioavailability and uptake of contaminants from compost.•Generally, biochar reduces harmful effect of feedstock on living organisms. Biochar is characterised by a large specific surface area, porosity, and a large amount of functional groups. All of those features cause that biochar can be a potentially good material in the optimisation of the process of composting and final compost quality. The objective of this study was to compile the current knowledge on the possibility of biochar application in the process of composting and on the effect of biochar on compost properties and on the content of contaminants in compost. The paper presents the effect of biochar on compost maturity indices, composting temperature and moisture, and also on the content and bioavailability of nutrients and of organic and inorganic contaminants. In the paper note is also taken of the effect of biochar added to composted material on plants, microorganisms and soil invertebrates.