Substituent constants and substituent effects in organoselenium and organotellurium compounds help chemists both explain and predict properties and reactivity ofthese organochalcogen compounds. In this chapter, recently reported substituent constants of selenium and tellurium‐containing groups are described, as are structure‐property and structure‐reactivity relationships in organoselenium and organotellurium compounds, particularly as they relate to the electronic and steric effects of substituents. The more quantitative examples of substituent effects on the properties and reactivity of chalcogen‐containing groups and molecules are presented, as are related papers that may lack the quantitative aspects, but still provide insight into the chemistry and properties of molecules. Selected series of compounds in which differences across the chalcogens can be seen are also included, where relevant.