Authors: U. Becciani, V. Antonuccio-DeloguFLY is a code that uses the tree N-body method, for three-dimensional self-gravitating collisionless systems evolution. FLY is a fully parallel code based on the tree Barnes-Hut algorithm and periodical boundary conditions are implemented by means of the Ewald summation technique. FLY is based on the one-side communication paradigm to share data among the processors, that access to remote private data avoiding any kind of synchronism. The current version 3.1 uses the MPI-2 standard. It uses the MPICH-2 library and can be used on any Linux PC cluster or on a single PC having MPICH-2 library. FLY 3.1 is freely available and open to improvements from contributors whose target is to enhance the code's capability, and/or porting it to other platforms. The code is written in Fortran 90. FLY is used to run simulations where only the gravitational forces are taken into account. FLY can communicate with other softwares based on the Paramesh structure, to consider the hydrodynamic effects, including other kinds of particles (the gas), to study the star formation and other hydrodynamical effects.PLEASE send an e-mail to Ugo Becciani when you download FLY.