The relationship between selenium and biology or medicine is a rather special one. For an element that gives rise to the amino acids selenocysteine and selenomethionine and is central to many physiological processes, surprisingly few selenium medications have ever reached the market. Nonetheless, the past decade or two has witnessed the emergence of a new generation of biologically active and pharmaceutically relevant selenium compounds, from the isolation of natural selenium nanoparticles produced by various bacteria and the identification of selenoneine in tuna fish to the synthesis of effective organoselenium compounds with anticancer and antimicrobial activity based on fairly exotic selenium motifs. This chapter therefore turns towards selenium compounds with redox modulating, often pro-oxidative, pro-apoptotic and eventually also lethal actions that exhibit antibacterial, antifungal, antimicrobial and anticancer activity. These compounds include selenium nanoparticles and simple selenium salts, seleninic acids, selenoureas and selenoesters, but also sophisticated, multifunctional selenium-based redox catalysts which promise a unique combination of efficiency and selectivity. From a chemical perspective, these agents usher in a new generation of reactive selenium compounds which provides impetus for an amazing synthetic selenium chemistry, where multi-components are sticking and azides are clicking, while the clock for many diseases may be ticking.