A critical review on the advancements in sol-gel method of doping TiO2 photocatalysts is provided. Various sol-gel and related systems of doping were considered, ranging from co-doping, transition metal ions doping, rare earth metal ions doping to other metals and non-metals ions doping of TiO2. The results available showed that doping TiO2 with transition metal ions usually resulted in a hampered efficiency of the TiO2 photocatalyst, though in some few cases, enhancements of the photocatalytic activity of TiO2 were recorded by doping it with some transition metal ions. In most cases, co-doping of TiO2 increases the efficiency of its photocatalytic activity. The review reveals that there are some elemental ions that cannot be used to dope TiO2 because of their negative effects on the photocatalytic activity of the catalyst, while others must be used with caution as their doping will create minimal or no impacts on the TiO2 photocatalytic efficiency. (C) 2009 Elsevier B.V. All rights reserved.