Fundamentals of the percolation theory and fractal geometry of aggregates a
re reviewed. The relationship between the fractal characteristics of aggreg
ates in dilute dispersion and percolation cluster was demonstrated. The phy
sical nature of the multiplicity of percolation thresholds in metal-filled
polymer films was discussed. The influence of the external effects on the p
osition of the percolation threshold and on the change of conductivity mech
anisms in a composite was considered.