Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT/PSS), available in the form of aqueous dispersion as colloidal particles, is one of the most successful conductive polymers. Because of its high electrical conductivity, transparency, and thermal stability, the PEDOT/PSS has potential applications to organic electronics such as capacitors, hole transport layers of organic light-emitting diodes, and transparent electrodes of touch panels and solar cells as an alternative of indium tin oxide (ITO). In this seminar, correlation between hierarchical structure and electrical conductivity of PEDOT/PSS will be investigated by means of XPS, XRD and C-AFM. The improvement of electrical conductivity can be achieved by hierarchical structure changes: crystallization of PEDOT molecules, removal of the insulating PSS from the surface of the PEDOT/PSS particles, and aggregation of the particles, affecting both intra- and inter-particle transport of charge carriers. Furthermore, if there is time, our recent work on synthesis and characterization of highly conductive PEDOT/PSS (> 1000 S/cm) and applications to flexible and stretchable organic devices will be presented.