Cytosine base editors (CBEs) and adenine base editors (ABEs), which are generally composed of an engineered deaminase and a catalytically impaired CRISPR-Cas9 variant, are new favorite tools for single base substitution in cells and organisms. In this review, we summarize the principle of base-editing systems and elaborate on the evolution of different platforms of CBEs and ABEs, including their deaminase, Cas9 variants, and editing outcomes. Moreover, we highlight their applications in mouse and human cells and discuss the challenges and prospects of base editors. The ABE- and CBE systems have been used in gene silencing, pathogenic gene correction, and functional genetic screening. Single base editing is becoming a new promising genetic tool in biomedical research and gene therapy.