The work has been concentrated on the optimization of the fabrication process of known solid state electrochromic devices and on the evaluation of their performance characteristics. Significant progress has been made in the successful routine fabrication of an improved structure consisting of SnO2-WO3- MgF2-Au-MgF2. This structure appears to have superior electrochromic properties when compared to the corresponding devices such as SnO2-WO3-SiO-Au which have been studied earlier. Although the lack of reproducibility of the electro- optical characteristics among samples prepared under apparently identical conditions still remains a major enigma, considerable insight has been obtained on various parameters which affect the performance of solid state electrochromics. The solid state electrochromic devices that are being investigated consist of a multilayer thin film structure. Each of the constituent layers can critically affect the electro-optical characteristics and therefore the major part of the work had to be directed toward the optimization of each layer with respect to parameters such as: substrate preparation, electrochromic material selection, methods of film preparation, control of deposition parameters such as rate of evaporation, substrate temperature, selection of suitable insulating material, development of a transparent counter electrode and the protection of the device against mechanical damage and chemical contamination.