Electrochromic windows can be described as devices based on electrochromic materials that work as reversible switching optical shutters, different device concepts and their electrochemical principles will be described. The relevance of electrochromic windows to low energy architecture will be discussed. A detailed analysis about the general requirements and the specific performances of electrochromic glazing for architectural applications will be introduced. The challenging issues of the present electrochromic technology approaching architectural glazing will be identified from the technological point of view and from the applications perspective. Emerging electrochromic glazing designs will be critically examined. A final discussion will be introduced about the different technical and marketing problems that slow down the time to market of electrochromic windows.