Conjugated polymers are semiconductors, which if doped can become conducting. Their electronic properties make them suitable for use in organic electronic devices such as transistors (OTFTs), light-emitting diodes (OLEDs) and solar cells (OPVs). The operating principles of these devices are discussed. Each of these devices have different requirements for their active materials. Among the important parameters which must be considered to optimise device performance are the energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) (known as the bandgap) which controls which colours of light can be absorbed or emitted, the energy levels of the HOMO and LUMO, which control the rate at which charges can be injected and extracted and the mobility of the charge carriers within the material. These parameters must be considered in designing or selecting suitable materials for use in these devices.