Windows are architectural elements that have been developed for hundreds of years and play a central role in house functionality and other environments where humans have developed their lives, transport, and work.

Smart windows are devices that, according to requirements, can dynamically modulate the transmittance of light. They are recognized as a promising technology for saving energy in buildings by controlling the indoor solar radiation. Therefore, smart windows have been considered indispensable devices to reduce the consumption of cooling energy, adjusting the transport of sunlight that allows it to pass on cooler, and preventing environmental heat on hot days. Important efforts have been made recently for the development of smart windows and the most advanced are electro, thermo, mechano, and photoresponsive. The electrochromic (EC) windows stand out, because they can be adjusted and integrated into the electrical system of buildings. The cornerstone of these windows is a functional material based on conjugated polymers, which can modulate the light via the modification of its optical properties. The major challenges for smart windows are the ability to operate at a low electrical potential, respond reasonably quickly to switching, be sufficiently transparent, have low thermal conductivity, stability against temperature variations, and a long service life.

This chapter discusses the main properties of the most relevant conjugated polymers that have EC characteristics, such as polythiophenes (PT), polypyrrole (PPy), polyanilines (PANI), polyfurans, and polycarbazoles (PCz). In addition, some examples will detail how a substituent introduction to the base polymer structure allows the optical and electrochemical properties to be modified to improve the performance of these materials into smart windows. Finally, a summary of the future perspectives is discussed.