Theory of tunneling conductivity in metal-containing nanocomposites has been developed. Three regimes of charge transport, corresponding to weak, strong, and super-strong applied electric fields were found to be characterized by different dependences of the nanocomposite's conductivity on the applied electric field and temperature. In the absence of external electric field, the metal nanoparticles become charged due to tunneling jumps between them. In case of weak electric field, the statistical distribution of positive and negative charges on nanoparticles is in quasi-equilibrium, and these charges make the dominant contribution to the charge transport. In the opposite case of strong and super-strong electric fields, the charge transfer consists of jumps of the electrons between neutral adjacent nanoparticles. In the strong field case, once the charges are created, they jump along or against electric field thus creating cumulative elementary charge displacement. The distribution of interparticle distances is derived based on distribution of their radii. This allows us to calculate the total tunneling current through the composite film as the function of the temperature and concentration of the nanoparticles. The magnetoresistance of ferromagnetic nanocomposites was found to be the same in cases of weak and super-strong electric fields, but differs from that in the strong field. © 2012 Wiley Periodicals, Inc.