This document aims to cover the most relevant mechanisms for the development of beam halo in high-intensity hadron linacs. The introduction will outline the various applications of high-intensity linacs and it will explain why, in the case of the CERN Superconducting Proton Linac (SPL)study a linac was chosen to provide a high-power beam, rather than a different kind of accelerator. The basic equations, needed for the understanding of halo development will be derived and employed to study the effects of initial and distributed mismatch on high-current beams. The basic concepts of the particle-core model, envelope modes, parametric resonances, the free-energy approach, and the idea of core-core resonances will be introduced and extended to study beams in realistic linac lattices. The approach taken is to study the behavior of beams not only in simplified theoretical focusing structures but to highlight the beam dynamics in realistic accelerators. All effects which are described and derived with simplified analytic models, are tested in realistic lattices and are thus related to observable effects in linear accelerators. This approach involves the use of high-performance particle tracking codes, which are needed to simulate the behavior of the outermost particles in distributions of up to 100 million macro particles. In the end a set of design rules will be established and their impact on the design of a typical high-intensity machine, the CERN SPL, will be shown. The examples given in this document refer to two different design evolutions of the SPL study: the first conceptual design report (SPL I) and the second conceptual design report (SPL II)
