This review paper summarizes data on mixed oxides containing niobium in an intermediate valence state, between 4+ and 5+. The major preparation procedures are considered, and the effects of different process parameters - composition of the starting reagents, temperature, pressure, duration, and environment - are analyzed from the viewpoint of the preparation of bulk, single-phase materials with niobium in a controlled formal oxidation state. The optimal synthesis conditions are established for reduced niobates with different structures. The oxidation behavior of the niobates(IV, V) is examined as a function of the structure type. The structural and electrical properties of the reduced niobates are shown to correlate with the carrier concentration (formal oxidation state of niobium). The major factors determining the carrier concentration in the niobates are identified. The results are used to establish the conditions under which a semiconductor-metal transition is possible in reduced niobates.