The unique properties of electrochromic (EC) colour switching have been widely observed from molecular EC materials to π-conjugated polymers (CPs). However, either their coloured or transparent state or both are generally unstable at voltage off state (V OFF ) losing their optical memory (OM). Therefore many efforts have been made to keep OM under V OFF . While the molecularly dissolved EC materials show poor OM due to the diffusion of the EC materials under V OFF , polymeric EC materials in thin film states afford a longer OM up to 200 s. The mechanism to reach a long OM at coloured and bleached states is fundamentally different. Therefore in order to reach optical memory at both coloured and transparent state under V OFF , called bistability, it is necessary to design polymers with a particular energy level as well as electrolytes. Ultimately, the side chain engineered ProDOTs show bistability in ionic liquids when their HOMO energy level is lower than the fermi level of the electrode. In this chapter we will review several approaches to achieve bistability in electrochromic devices as well as windows, in particular, focusing on the mechanism for bistable electrochromism in polymer thin films and current status of the related research.