Polymer nanocomposites hold promise for applications in electronic industry such as embedded capacitors, cellular phones, electromagnetic shielding, sensors and actuators. A high dielectric constant or appropriate conductivity is usually needed for such appplications. This can be reached by proper selection of the filler/matrix materials and processing conditions. In this research, polymer nanocomposites with carbonaceous fillers were designed and developed with controlled electrical properties. The nanocomposites were fabricated by solution mixing or melt-compounding method. One dimensional carbonaceous nanofillers with large aspect ratios such as carbon nanofiber (CNF), multi-walled carbon nanotube (MWNT), and carboxyl functionalized MWNT (FMWNT) were used as electrically conductive fillers for polymer nanocomposites. Polyvinylidene fluoride (PVDF) and high density polyethylene (HDPE) were selected as the polymer matrices. The dielectric and conducting properties of these nanocomposites were systematically studied over a wide range of frequencies (40 Hz-110 MHz) The study of dielectric properties was mainly focused on proper filler selection and process innovations to increase the dielectric constant sufficiently but to reduce the loss tangent. To enhance the overall dielectric performance, we firstly added BaTiO3 (BT) nanoparticles to polymer matrix. The dielectric constant and loss tangent of the PVDF/BT nanocomposites depended greatly on the nanoparticle concentration. The dielectric constant increased with filler concentration, while the loss tangent showed a reverse tendency. Additions of low loadings of MWNTs to the BT/PVDF nanocomposites created a three-phase hybrid system with enhanced dielectric constant. The conduction behavior of polymer nanocomposites containing carbonaceous nanofillers is considered of technological importance. This study aims to clarify the mechanisms underlying the electrical conduction of such nanocomposites. Both the AC and DC conductivity and their dependences on temperature and electric field were investigated. The AC conductivity of polymer nanocomposites was analyzed to yield information relating the connectivity of conducting clusters within composites. The AC conductivity of polymer nanocomposites beyond percolation threshold can be well characterized by DC plateau, characteristic frequency and critical exponent x. The characteristic frequency is related to the size of conducting clusters. Its variation with temperature or electric field reflects the effect of these two factors on the conductive filler connection in the nanocomposites. The exponent x is dependent on the frequency and temperature, but almost independent of electric field. The DC conductivity was found to follow a typical percolating behavior. The conduction of charge carriers in these percolating composites occurred mainly through the interconnected fillers in the polymer matrix. It is also showed that the HDPE based nanocomposites exhibit a positive temperature coefficient effect of resistance. Under the application of an external electric field, two competiting processes, i.e. internal field emission and joule heating occurred in the nanocomposites were investigated. The former enhanced the conductivity and resulted in nonlinear conduction behavior, while the latter decreased the conductivity and induced electrical conduction relaxation.