Ternary GeSiSn alloys have been recently demonstrated on Ge- and GeSn-buffered Si substrates These alloys, with a two-dimensional compositional space, make it possible to decouple lattice constant and electronic structure for the first time in a group-IV system This paper reviews the basic properties of the GeSiSn alloy. presents some new results on its optical properties, and discusses the approach that has been followed to model heterostrcutures containing GeSiSn layers for applications in modulators, quantum cascade lasers, and photovoltaics. (c) 2009 Elsevier B.V. All rights reserved