Polydiacetylenes (PDAs) are conjugated polymers with the general structure [Note: Equation omitted. See the image of page 51 for this equation.], which can be obtained as macroscopic crystals consisting of fully extended polymer chains. The factors that control the solid-state polymerization of disubstituted diacetylenes and the perfection of the resultant polymer crystals are discussed. The presence of large substituent groups leads to weak interchain coupling so that PDAs are quasi-one-dimensional materials. Optical spectroscopy has shown that the strong dichroism of PDA crystals results from an excitonic excited state lying about 0.5 eV below the interband transition. The latter is weakly absorbing but can be observed by electro-modulation spectroscopy. Thermal and photogeneration of free carriers is inhibited by the large band gap (greater than 2 eV), the weak interband absorption and the high probability of geminate recombination. The dependence of photocurrents on applied field is adequately described by the Onsager theory of geminate recombination in one dimension. This leads to the conclusion that carrier drift velocities are saturated even for low applied fields. In addition, photocurrent decay displays anomalous behaviour. Both phenomena have been attributed to scattering by defects, which leads to the failure of linear response theory in one dimension. Even for imperfect samples with high trap densities the carrier motion is still accurately described by a one-dimensional diffusive model. This predicts unusual field and temperature dependencies in the decay of photocurrents that have been confirmed experimentally.