Biosensor is an analytical device that couples a biological element, such as a biopolymer or microorganism capable of sensing the target analyte, with a physicochemical signal transducer to enable rapid, accurate and sensitive detection. The biological element determines the specificity of the biosensor and significantly contributes to its sensitivity. In this paper we review heavy metal biosensors with emphasis on their biological components – enzymes activated or inhibited by heavy metal ions, metal-sequestering peptides, non-enzymatic proteins, DNAs possessing metal-dependent enzymatic activity, antibodies against metal chelates, metal-dependent transcription activators/repressors combined with their target DNA sequence as well as whole bacterial cells, both natural and genetically engineered by introducing the metal-responsive transcription regulation elements controlling the expression of a reporter gene.