Abstract: In eukaryotes, Ca2+ is an important second messenger and regulates diverse cellular activities ranging from muscle contraction to fertilization. In bacteria, growing evidence suggests that Ca2+ also plays important regulatory roles in various physiological processes. Here we review current understanding of calcium regulation in bacteria from the following aspects: 1） the concept of bacterial [Ca2+]i and its determination; 2）cellular processes affected by [Ca2+]i changes; 3）transportation of Ca2+ across bacterial membrane; 4）eukaryotic calcium binding proteins in bacteria and their functions.