The experimental and theoretical studies of lepton deep-inelastic scattering on nuclear targets are reviewed. The main problems which must be solved in analyzing the x and A dependences of the deep-inelastic cross sections are outlined. Several of the models proposed for studying the contribution of nuclear effects to the structure functions F 2 (x) are analyzed. A relativistic field-theoretical approach to studying lepton deep-inelastic scattering on very light nuclei is developed. The modifications of the nucleonstructure in D, 3 H , 3 He , and 4 He are studied for the first time within a unified approach. It is found that the modifications evolve in a manner completely different from that observed earlier for heavy nuclei. In particular, it is found that the pattern of F 2 (x) modifications represented in terms of the ratios F 2 A /F 2 N ( D ) is determined by the values (1−x 3 )=0.32 (D/N), 0.16 ( 3 He/D ), and 0.08 ( 4 He/D ). These results allow the definition of the class of modifications of the bound-nucleon structure and the introduction of the unit of nucleon-structure modification. Theoretical justification of the concept of two-stage evolution of the nucleonstructure as a function of A, the first stage occurring for A⩽4 and the second for A>4, is also obtained. The long-standing problem of the nature of the EMC effect is explained as a modification of the nucleonstructure in the field of the nuclear forces in a three-nucleon bound system.