BETL is a C++ template library for the discretisation of boundary integral operators as they arise in various physical and engineering applications. Prominent examples are, e.g., electrostatic or thermal models as well as the scattering of acoustic and electromagnetic waves. While BETL currently implements the discretisation of 3-dimensional boundary integral operators via Galerkin schemes its design principles allow also for the incorporation of other discretisation schemes such as, e.g., the still popular collocation methods. Over the years this project has grown slightly bigger than it was originally expected. In its original form BETL was just intended to serve as set of methods with rather limited functionality for maintaining an existing industrial Boundary Element solver. Since then BETL has become a fully autonomous tool on which powerful Boundary Element applications can be build. BETL’s strength lies in its use of well-known design principles in conjunction with state of the art C++ language features. This ensures the fast development of robust, extendable, and reliable numerical schemes and implementations which are somehow related to the discretisation of boundary integral operators