Since the 70's and the discovery of quarks, our understanding of the structure of the nucleon has greatly improved. The theory of the strong interaction, QCD, allows for an accurate description of numerous experimental results. However, some aspects of nucleon structure, especially the spin structure, are not yet well understood. The COMPASS experiment, located at CERN on the SPS, uses a longitudinally polarized muon beam that scatters off a longitudinally polarized LiD and NH_3 target to probe the internal structure of the nucleon. These results are used in a next-to-leading-order (NLO) QCD fit with the world data to obtain the polarized parton distribution functions. An evaluation the Bjorken sum rule, that connects the integral of the non-singlet spin-dependent structure function with the ratio of the weak coupling constants, will be given using the COMPASS data alone. To complement these results, the double longitudinal spin asymmetries A_LL(p_T) for single hadrons with large transverse momentum (p_T>1 GeV/c) in the photo-production regime are presented. These asymmetries are particularly sensitive to photon-gluon fusion and thus to the gluon polarization. They supply new inputs for global fits based on NLO collinear QCD calculations. Finally, results on hadron, pion and kaon multiplicities will be presented. They are essential quantities to extract the fragmentation functions of quarks into hadrons, needed in the description of various processes where factorization holds.