Electrochromism (reversible color change of a material, induced by an electric field) has been investigated in tungsten trioxide thin films. It is known that electrochromic device lifetime is generally limited by corrosion of WO3 films in an acid electrolyte. Two mechanisms of corrosion were observed, viz., general dissolution and interfacial delamination. It was found that the dissolution of WO3 films in acid could be attributed to high concentration of thermodynamically unstable species (W02 and W2O5) in the as-deposited films. These species were identified by X-ray photoelectron spectroscopy data and are consistent with Rutherford backscattering spectroscopy data which showed an oxygen deficiency (0/W = 2.76). The Pourbaix diagram for tungsten indicated that WO3 was the thermodynamically stable specie for the present storage condition (pH = 0.5, E = 0.4 Vshe). A corrosion mechanism was proposed consisting of dissolution of WO2 and W2O5 and precipitation of crystalline WO3 and its hydrates. Interfacial delamination occurred when WO3 and its hydrates precipitated back onto the original films. The oxygen content in the WO3 films was increased by oxygen backfilling during evaporation. Dissolution and interfacial delamination of the oxygen enriched films were reduced to negligible rates due to reduced concentration of WO2 and W2O5 . However, the electrochromic properties were degraded by oxygen enrichment. For example, increased resistivity and decreased optical efficiency in the oxygen enriched films resulted in slower coloration speed. The resistivity increase and decreased optical efficiency were explained by postulating an increased density of inactive electron trapping sites. The porosity of the films could be increased by deposition at high background pressure, resulting in increased surface area and absorbed water. The bleaching speeds and self-erasure rates were increased since the rates of removal of protons were increased by the increases in porosity and absorbed water. In another approach to increase the electrochromic device lifetime, the electrolyte was modified. Devices using a solution of LiClO4 in propylene carbonate exhibited excellent lifetime. Switching speeds were increased by increased porosity, deposition of MgF2 overlayers, and more conductive indium-tin-oxide layers. In addition, solid state electrochromic devices using a hydrated MgF3 film were fabricated.