The DLR FlexibleBody library to model large motions of beams and of flexible bodies exported from finite element programs. The new DLR FlexibleBodies library enables and supports the object-oriented and mathematically efficient modelling of flexible bodies as components of mutlibody and of arbitrary physical systems. It provides Modelica model classes to model (a) beams and (b) general flexible bodies exported from finite element programs. The motion of a flexible structure is defined by superposition of a in general large, non-linear motion of a reference frame with small electric deformations. This paper gives an overview on the background, concepts and ideas on which the library is based and how the Modelica user may take advantage of it.