[en] The structure of the proton and the origin of the proton spin has been a puzzle for many years. The EMC collaboration at CERN provided the first experimental data on the spin structure of the proton. The result was almost zero net contribution from quarks. Over the past 20 years new measurements of polarized parton distributions became available. The present value of the quark contribution to the proton spin is one third. The remaining 60 percent of the proton spin come from the gluons and orbital angular momentum of quarks and gluons. We investigate how the spin of the proton originates from the spin of its constituents. We study the proton using the phenomenologically accessible parameters such as distribution functions for quarks and gluons. The basic understanding of the proton structure (and in particular its spin structure) is important for interpreting the results of the LHC, which in turn can be used to refine the present knowledge. The proton spin structure gives a detailed information about the dynamical structure of the proton. Based on the present experimental data we suggest that the gluons and quarks play equally important role in the structure of the proton. (orig.)