In the last quarter of the 20th century, the pace of structural change in the equity markets has accelerated dramatically and, as it has, regulation has come to play an increasingly central role in the development of market structure. The purpose of this text is to consider regulation’s contribution to the efficiency of the US equity markets. Sharply different opinions are expressed on the matter, as the discussion ranges from Congressional oversight, to SEC involvement in market structure issues, to the self-regulatory responsibilities of the market centres, most notably, the New York Stock Exchange and the Nasdaq Stock Market.