Serpent is a continuous-energy multi-purpose three-dimensional Monte Carlo particle transport code. It is under development at VTT Technical Research Centre of Finland since 2004. Serpent was originally known as Probabilistic Scattering Game (PSG) from 2004 to the first pre-release of Serpent 1 in October 2008. The development of Serpent 2 was started in 2010. The current stable version Serpent 2.2.0 was released in May 2022.