This thesis concerns experimental studies of nanocrystalline tungsten trioxide thin films. Functional properties of WO3 have interesting applications in research areas connected to energy efficiency and green nanotechnology. The studies in this thesis are focused on characterization of fundamental electronic and optical properties in the semiconducting transition metal oxide WO3. The thesis includes also applied studies of photocatalytic and photoelectrochemical properties of the material.     All nanocrystalline WO3 thin films were prepared using DC magnetron sputtering. It was found that structures like hexagonal and triclinic phase with different properties can be produced with sputtering technique. Thin film deposition has been performed using different process parameters with emphasis on sputter pressure and films that mainly consist of monoclinic γ-phase, with small contributions of ε-phase. Changes in the pressure are shown to affect the number of oxygen vacancies in the WO3 thin film, with close to stoichiometric WO3 formed at high pressures (30 mTorr), and slightly sub-stochiometric WO3-x, x = 0.005 at lower pressures (10 mTorr). Both stoichiometric and sub-stoichiometric thin films have been characterized by several structural, optical and electronic techniques.    The electronic structure and especially band gap states have been explored and optical properties of WO3 and WO3-x have been studied in detail. The band gap has been determined to be in the range 2.7-2.9 eV. Absorption due to polaron absorption (W5+  -W6+), oxygen vacancy sites (Vo -W6+), and due to differently charged oxygen vacancy states in the band gap have been determined by spectrophotometry and photoluminescence spectroscopy, in good agreement with resonant inelastic x-ray spectroscopy and theoretical calculations. The density of electronic states in the band gap was determined from cyclic voltammetry measurements, which correlate with O vacancy concentration as compared with near infrared absorption.      By combining different experimental methods a thorough characterization of the band gap states have been possible and this opens up the opportunity to tailor the WO3 functionalities. WO3 has been shown to be visible active photocatalyst, and a promising electrode material as inferred from photo-oxidation and water splitting measurements, respectively. Links between device performance in photoelectrochemical experiments, charge transport and the electronic structure have been elucidated.