This chapter discusses the metabolism and biotechnological applications of heavy metals, emphasizing their toxicity and the mechanisms by which microorganisms resist them. Heavy metals, defined as elements with a density above 5 g/cm³, are essential trace elements in some cases but can be toxic at higher concentrations. The chapter outlines the basic principles of homeostasis for heavy-metal ions and compares the transport of 17 important heavy metals. It highlights the two main types of uptake systems: fast, unspecific systems driven by the chemiosmotic gradient, and slow, specific systems that use ATP hydrolysis and are inducible. The text also explains how heavy metals can bind to SH groups and other physiological ions, leading to enzyme inhibition and toxicity. Finally, it mentions the role of mutations in reducing the expression of fast, unspecific transporters, making cells more tolerant to heavy metals.This chapter discusses the metabolism and biotechnological applications of heavy metals, emphasizing their toxicity and the mechanisms by which microorganisms resist them. Heavy metals, defined as elements with a density above 5 g/cm³, are essential trace elements in some cases but can be toxic at higher concentrations. The chapter outlines the basic principles of homeostasis for heavy-metal ions and compares the transport of 17 important heavy metals. It highlights the two main types of uptake systems: fast, unspecific systems driven by the chemiosmotic gradient, and slow, specific systems that use ATP hydrolysis and are inducible. The text also explains how heavy metals can bind to SH groups and other physiological ions, leading to enzyme inhibition and toxicity. Finally, it mentions the role of mutations in reducing the expression of fast, unspecific transporters, making cells more tolerant to heavy metals.