Application of the researcher's tools to the gurgly, quicksilver interactions between mother and infant seems exceedingly difficult, even somewhat risible, but Stern is no casual observer and his work shows originality and full appreciation of intuitive responses. Specifically, he examines repertoires of facial and vocal behaviors for evidence of perceptual abilities and patterns of affective responses. Developmental landmarks--smiling, laughing, anticipating the tickler's next move--are seen as part of intricate mutual feedback systems which emerge without prompting and usually manage to regulate themselves. From these episodes of shared engagement, a relationship evolves; Stern suggests ""a basic process unit of interactive experience"" (a split-second or few-second exchange) as the basis for this accumulating sequence--a useful conceptual perspective. The tone of these exchanges is important because the attachment behavior of the first year influences separation and individuation in the next; over- or understimulation--""missteps in the dance""--can distort personality formation. Although this relies on lab language (infant-solicited social behavior, selective paradoxical responsivity), which diminishes its value for parents, it is thoroughly suitable--and quite helpful--for those working with infants and their mothers.