During a lecture on the optical properties of gold in 1857, Michael Faraday remarked that “a mere variation in the size of its particles gave rise to a variety of resultant colors.” Since Faraday was referring to his studies of nanometer-scale gold particles (now commonly referred to as nanocrystals), it is clear that the topic of this issue of MRS Bulletin is an old field with an important history. Indeed, the study of Faraday's gold sols was critical for the development of several aspects of modern materials science, including how light interacts with matter and how colloids can be formed, manipulated, and controlled.