Electrochromic devices for use as switchable architectural glazing have developed rapidly over the past decade. There are now several companies with marketable (or near market) products. of quite substantial size. There is also a wide variety in the designs of the devices used in these products, in particular, the ionic conductor and ion storage layers. As a result, these devices exhibit some quite different switching characteristics, and it is still unclear what the long-term life and degradation behaviour of these devices will be. There are many open questions about how to control the devices in real operating conditions, where the temperature will be significantly above room temperature, and what the effect of increasing the area is on the lifetime. This paper reviews the way in which electrochromic devices switch and introduces models for all aspects of the device performance, in particular, the effects of electrical conductivity of the transparent electrodes, the ionic conductivity of the ion conductor, the device size and the temperature of operation. The application of these models to the design of large area electrochromic windows for architectural applications is then discussed and some examples of the use of the models to understand device behaviour and to determine how best to switch devices are presented.