The paper presents a novel mechanism for electrical conduction in disordered materials, known as fluctuation-induced tunneling. This mechanism is characterized by the role of thermally activated voltage fluctuations across insulating barriers in determining the temperature and field dependencies of conductivity. The author derives a theoretical expression for the tunneling conductivity, which exhibits thermally activated behavior at high temperatures and becomes temperature-independent at low temperatures. The shape of the tunneling barrier controls the temperature dependence of the conductivity between these two extremes. An expression for the high-field tunneling current is also derived, showing that the tunneling current increases nonlinearly with the field, but the degree of nonlinearity decreases with increasing temperature. The theory is generalized to a random network of tunnel junctions using effective-medium theory. The predictions of the theory are compared with experimental results for three disordered systems: carbon-polyvinylchloride composites, heavily doped GaAs, and doped polyacetylene, showing excellent agreement. The nonmetallic temperature dependence of resistivity in doped metallic polyacetylene samples is explained using the present theory.The paper presents a novel mechanism for electrical conduction in disordered materials, known as fluctuation-induced tunneling. This mechanism is characterized by the role of thermally activated voltage fluctuations across insulating barriers in determining the temperature and field dependencies of conductivity. The author derives a theoretical expression for the tunneling conductivity, which exhibits thermally activated behavior at high temperatures and becomes temperature-independent at low temperatures. The shape of the tunneling barrier controls the temperature dependence of the conductivity between these two extremes. An expression for the high-field tunneling current is also derived, showing that the tunneling current increases nonlinearly with the field, but the degree of nonlinearity decreases with increasing temperature. The theory is generalized to a random network of tunnel junctions using effective-medium theory. The predictions of the theory are compared with experimental results for three disordered systems: carbon-polyvinylchloride composites, heavily doped GaAs, and doped polyacetylene, showing excellent agreement. The nonmetallic temperature dependence of resistivity in doped metallic polyacetylene samples is explained using the present theory.