This paper reviews the physics of the spallation which is a nuclear reaction in which a particle (e.g. proton) interacts with a nucleus. Given to the high energy of the incident proton, in a first stage it interacts with the individual nucleons in an intranuclear cascade which leads to the emission of secondary particles (neutrons, protons, mesons, etc.). In a secondary stage the nucleus is left in an excited state and can de-excite by evaporation and/or fission. Given to the high number of secondary neutrons produced (∼30 n/p for proton energy of 1 GeV), this reaction can be used as a source of neutrons, for example for ADS systems as external source to drive the sub critical reactor. The main codes used in the ADS target design and an example on the utilization of one of these codes (the LAHET code) for typical ADS target are given.353 B894897Gudowsky, W., (1999) Nuclear Physics A, 654, p. 436Baylac-Domengetroy, V., (2003) Investigation Related to the Generation of Reaction Products in the Target of Accelerator Driven Systems for Nuclear Waste Incineration, , FZKA publication, DecemberKarol, P.J., Nucleus-nucleus reaction cross-section at high energies: Soft sphere model (1974) Phys. Rev. C, 11 (4)Cole, A.J., Proton induced spallation reactions between 300 MeV and 20 GeV (1987) Phys. Rev. C, 36 (4)Strugalski, Z., (1996) Spallation Mechanism and Characteristics, , publication of the Institute of Physics of Warsaw, PolandSerber, R., The production of high energy neutron by stripping (1947) Physical Review, 72 (11). , DecemberCugnon, J., Proton-nucleus interaction at high energy (1987) Nuclear Physics, A462, pp. 751-780Prael, R., User guide to LCS: The LATE code system Los Alamos National Laboratory, Report, LA-UR-89-3014http://www.fluka.orgWater, L.S., (1999) MCNPX Users Manual, , TPO-E83-G-UG-X-00001, revision 0, NovemberBriesmeiter, J.F., (1997) MCNP - A General Monte Carlo N-particle Transport Code, , LA-12625-M Version 4B, UC705 and UC700, MarchDeppman, A., A Monte Carlo method for nuclear evaporation and fission at intermediate energy (2003) Nucl. Instr. and Meth B, 211, pp. 15-21Anéfalos, S., (2005) Development of Crisp Package for Spallation Studies and ADS (Accelerator Driven System), , accepted for publication at Nuclear Science end EngineeringAnéfalos, S., The utilization of crisp code in hybrid reactor studies (2004) XXVII Reunião de Trabalho Sobre Física Nuclear No Brasil, , September 07 -11, Santos, SPTichelen, K.V., MYRRHA project, a windowless ADS design (1999) Proc. ADATTA 99, , Praga, Czech RepublicAnsaldo, XADS Pb-Bi cooled experimental accelerator driven system reference configuration (2001) Summary Report