This dissertation reports an investigation into context effects in conservation of number and quantity research. During the past ten years, a number of studies have shown that manipulation of the social or linguistic context in which the conservation transformation is set may increase the tendency of children below six or seven years to give conserving responses. These studies have often been taken as evidence that conservation competence is present in children younger than previously believed. Such 'early competence' positions are criticized in the dissertation. It is argued that the adoption of mental structures as 'competence' explanations for performance in cognitive tasks has led to a problem in conceiving of development, whereby interactionist accounts of development tend to reduce to maturational ones. Reconceptualizing the object of study of developmental psychology as the child's induction into, and participation in social practices, allows the reintroduction of the idea of development. The study of context assumes a central position in such an enterprise. Existing context studies are reviewed but it is concluded that they have, as yet, failed to exploit this potential. The experimental sections of the dissertation provide support for the theoretical position taken. Results from experiments that replicate existing context ef f ects, with added controls, and from experiments designed to avoid the methodological difficulties of the standard and more recent modified tasks, all point to the conclusion that when the child constr ues the conservation task as concerned with quantification, there is a serious tendency to confuse number or quantity with aspects of the appearance of the display. Changing the context from one of quantification to one of reward for helping an adult, changes the way the child construes the problem and results in increased conserving responses. This result is related to the child's understanding of the relative term, 'more'. It is concluded from the theoretical and experimental work presented, that viewing the child's understanding of number and quantity as constructed through participation in social practices avoids many of the difficulties of earlier cognitivedevelopmental accounts of conservation and provides solutions to many of the problems of conservation research. The final chapter outlines a tentative theory of conservation development based on recent attempts to theorize