[en] We review theoretical work done in studies of the Flavour Changing Neutral Current (FCNC) B-decays in the context of the Standard Model. Making use of the QCD-improved effective Hamiltonian describing the so-called vertical stroke ΔBvertical stroke =1 and vertical stroke ΔBvertical stroke =2, vertical stroke ΔQvertical stroke =0 transitions, we calculate the rates and differential distributions in a large number of B-decays. The FCNC processes discussed here include the radiative decays B → X_s + γ, B → X_d + γ, and the semileptonic decays B → X_s l⁺l^-, B → X_dl⁺l^-, B → X_s ν_l anti ν_l, and B → X_d ν_l anti ν_l. We also discuss the inclusive photon energy spectrum calculated from the Charged Current (CC) decays B → X_c + γ and B → X_u + γ and the mentioned FCNC radiative decays. The importance of carrying out measurements of the inclusive photon energy spectrum in B-decays is emphasized. Using phenomenological potential models and the Heavy Quark Effective Theory (HQET) we estimate decay branching ratios in a number of exclusive FCNC B-decays. Purely leptonic and photonic decays (B_d, B_s) → l⁺l^- and (B_d, B_s) → γγ are also estimated. The principal interest in the studies of FCNC B-decays lies in their use in determining the parameters of the standard Model, in particular the CKM matrix elements and the top quark mass. The parametric dependence of these and other QCD-specific parameters on the rates and distributions is worked out numerically. (orig.)