This chapter explores the mechanisms of self-healing in concrete through Microbiologically Induced Calcite Precipitation (MICP), detailing the chemical reactions involved in the biomineralisation process. It examines the sequence of microbial reactions that lead to the formation of mineral deposits, ultimately enhancing the durability and longevity of concrete. The chapter begins by outlining the fundamental principles of biomineralisation that drive calcite precipitation and contribute to the healing of cracks. It also explains the biological mechanisms underlying the formation and development of mineralised structures. A wide range of bacterial species previously investigated for this purpose is discussed. Finally, the chapter analyses the various factors influencing the biomineralisation process, with a particular focus on the mechanisms involved in crack filling.