Introduction Although some degree of shyness is experienced by most children during development, a small percentage (10-15 per cent) of children are consistently anxious, quiet, and behaviourally inhibited during social situations, in particular unfamiliar social situations (see Kagan, Chapter 2, this volume). Many of these children are characterized by a distinct pattern of physiological responses during resting conditions and in response to social challenge (see Kagan, Reznick and Snidman, 1987, 1988; Schmidt, Fox, Sternberg, Gold, Smith and Schulkin, 1999; Schmidt, Fox, Schulkin and Gold, 1999; Schmidt, Polak and Spooner, in press, for reviews) and some of them may be at risk for anxiety and internalizing-related problems (e.g., depression, social withdrawal) during early development (see e.g., Hirshfeld et al., 1992; Rubin, Stewart and Coplan, 1995) and adjustment problems in later years (Bell et al., 1993; Caspi, Elder and Bem, 1988; Schmidt and Fox, 1995; Zimbardo, 1977).