Rheolef is a computer environment that serves as a convenient laboratory for computations involving finite element methods. It provides a set of unix commands and C++ algorithms and containers. In particular, this environment allows the user to express a partial derivative problem in terms of finite element spaces, discrete fields, bilinear forms, geometries and meshes. Rheolef is the only one environment to our knowledge that bases on such powerful variational concepts as basic data structures. This set of data structure is completed by the most up-to-date algorithms: preconditioned sparse solvers for incompressible elasticity, Stokes and Navier-Stokes flows, characteristic method for convection dominated heat problems, \ldots All these brick can be combined, as a Lego game, in order to solve more complex nonlinear problems: the library provides also nonlinear generic solvers such as damped Newton methods.