Considerable research effort has been devoted to discovering and mapping out children's understanding of mental phenomena. Developmental psychologists have assessed children's ability to attribute mental states as well as their tendency to explain and predict human actions and emotions in terms of pre-existing mental states. Based on empirical findings obtained to date, it is believed that children come to understand human desires before they acquire a similar understanding of beliefs. In addition, it has been suggested that most children under the age of 3 years operate without any conception of belief (Wellman, 1993; Wellman & Woolley, 1990).  The present study which consisted of three experiments aimed to test young children's understanding of beliefs and desires, with an emphasis on how these states relate to a person's actions, emotions, and perceptual experience. Experiment 1 examined 18- to 30 month-old infants' understanding of the link between seeing and knowing, and their understanding of the link between surprised reactions and belief violations. Experiments 2 and 3 were created to assess 18- to 30-month-old children's understanding that desires guide actions and the knowledge that happy and sad reactions depend on the outcome of a person's pre-existing desires. In order to assess knowledge of these relationships, toddlers were administered a modified version of the preferential looking task that included videotaped stories involving actors and objects. The results of this series of experiments suggested that children as young as 18 months are "desire-psychologists" in the sense that they are able to relate another person's actions and emotional responses to that person's pre-existing desires. In contrast, children of that age showed no recognition that surprised reactions are associated with violations in a person's knowledge state, and no understanding of the link between seeing and knowing. Because of children's unanticipated bias for looking at the surprised expression, it is not possible to conclude presently whether children under 3 operate without a conception of beliefs.