The EIRENE neutral gas transport Monte Carlo code has been developed initially for TEXTOR since the early 1980s. It is currently applied worldwide in most fusion laboratories for a large variety of different purposes. The main goal of code development was to provide a tool to investigate neutral gas transport in magnetically confined plasmas. But, due to its flexibility, it also can be used to solve more general linear kinetic transport equations by applying a stochastic rather than a numerical or analytical method of solution. Major applications of EIRENE are in connection with plasma fluid codes, in particular with the various versions of the B2 two-dimensional plasma edge fluid code. The combined code package B2-EIRENE was developed, again initially for TEXTOR applications, in the late 1980s. It too has become a standard tool in plasma edge science. It is currently mainly used for divertor configurations, such as by the ITER central team, to assist the design of the ITER divertor. Both the EIRENE and B2-EIRENE concepts are introduced and illustrated with sample applications.