The in situ formation of conductive carbon lines in polymeric substrates was demonstrated by “writing” with CO2 and Nd:YAG lasers. The formation of conductive carbon lines on polymer substrates proceeds through a series of steps including; absorption of light, initiation of decomposition reactions, and thermal propagation of the pyrolysis. The effect of material composition, power density, energy density, and wavelength on the electrical resistance and morphology of the carbon lines was investigated. Above a critical laser energy density (= 400 J/cm2) sufficient light is absorbed by the substrate such that initiation of the pyrolysis reactions occurs. The decomposed polymer strongly absorbs the incident radiation and the thermal propagation of the reactions forms shiny, conductive, carbon lines. The dimensions of these lines are dependent on the energy density impinging on the polymer and are independent of the wavelengths investigated. If too high a power density is employed (∼ 104 Watts/cm2), an ablated track is formed down the center of the carbon line. This results in the relative insensitivity of the linear resistance with increasing power above 104 Watts/cm2.