High strength steels are affected by hydrogen embrittlement (HE) that can be a cause of their failure. A combination of theoretical methods and experimental technics should provide a better understanding of the mechanisms related to this phenomenon. In this thesis, a number of aspects related to HE in bcc iron are studied from a theoretical point of view in the framework of density functional theory. The results of theoretical investigations are compared to available experimental results when possible. At the current stage of knowledge, there is no universal explanation of H behaviour in steels. However, a number of HE mechanisms have been proposed in literature as the most likely to be operative in steel: HEDE (hydrogen-enhanced decohesion) and HELP (hydrogen enhanced localized plasticity). In this thesis, the key elements of these mechanisms, like decohesion of interatomic bonding or hydrogen accumulation at structural defects like dislocations and grain boundaries, are studied in detail at the atomic level. Firstly, the ground state of the mixed M111 dislocation was investigated in bcc iron and compared with other metals. M111 dislocations exhibited a special core structure and comparably large Peierls stresses in comparison to the 1/2 <111> screw dislocations. Two types of core structures were investigated and the bond-centered structure was found to be the ground state for all metals. For Fe, however, the energy difference between the two possible structures was found to be minimal. Further, H trapping behaviour for a number of common defects in bcc iron such as vacancies, grain boundaries (GB), free surfaces (FS) and dislocations of two aforementioned types (screw and mixed) has been described in detail. H trapping profiles at all considered defects have been obtained and the strongest trapping energies directly at the center of the defects have been found. Investigated vacancies and GBs have been identified as more thermodynamically stable (deeper) traps than dislocations. At the same time, from the estimation of McLean-Langmuir segregation isotherm, all defects have been found to have virtually the same concentrations of trapped H atoms at the room temperature +/- 100K. Carbides play an important role in HE as potential traps for H. Thus, H trapping behavior at the number of MeC/Fe interfaces (Me= Ti, V, Nb) has been studied. According to the results, H is most likely trapped at the tetrahedral interstitial sites in the distorted lattice of ferrite at the MeC/Fe interface. Carbon vacancies are not thermodynamically stable at coherent MeC/Fe interfaces, and therefore may play no role in H trapping there. However, the results show that vacancies in carbides can still be very attractive sites for H if it can get inside of the bulk of carbides avoiding the high energy barrier separating Fe and MC at the interface. Since the estimation of the bulk and GB cohesion in iron for the elements is important to the understanding of their effect on HEDE, such calculations have been carried out for the majority of elements from the periodic table. Based on the partial cohesive energies and strengthening energies indicating influence of the elements on iron bulk and GB cohesion respectively, a list of the elements, which can decrease negative H effect, has been obtained. Finally, the interactions of the elements from the aforementioned list with H in Fe bulk and at Fe GB have been investigated and the most effective alloying elements in H resistant steels have been selected. This thesis is devoted to theoretical density functional theory investigation of HE in bcc iron at the atomic scale taking into account aspects of several HE mechanisms. In the framework of this work, a unique set of data on H trapping energies and H interaction with some alloying elements that can be used in the design of new H resistant steels has been obtained.