Thermochromic VO2-based films have higher infrared transmittance below a "critical" temperature tau(c) than above this temperature and can be used for regulating the solar energy transmittance T-sol in energy efficient windows. Pure VO2 is not practical, though, since (i) the modulation of T-sol is too small at tau(c), (ii) the luminous transmittance is too low, and (iii) tau(c) is too high. This paper discusses how these three challenges can be met. Specifically, using VO2-based nanoparticle composites rather than films makes it possible to significantly alleviate (i) and (ii), Mg doping of VO2 can give further improvements of (ii), and W doping (and to some extent also Mg doping) is important for (iii) and can bring tau(c) to a comfort temperature. The paper hence delineates a path towards practically useful thermochromic fenestration. (C) 2011 Elsevier B.V. All rights reserved.