Abstract Introduction Historical Outline Chemical Structures Hydrogen Sulfide and Inorganic Polysulfides Chemistry, Structures and Characterization Occurrence Metabolism under Oxidative Conditions Metabolism under Reductive Conditions Elemental and Biologically Produced Sulfur Chemistry, Structures and Characterization Occurrence Metabolism under Oxidative Conditions Metabolism under Reductive Conditions Polythionates Chemistry, Structures and Characterization Occurrence Metabolism under Oxidative Conditions Metabolism under Reductive Conditions Organic Polysulfanes Chemistry, Structures and Characterization Occurrence Metabolism under Oxidative Conditions Metabolism under Reductive Conditions Summary Outlook and Perspectives Patents