<p>Peer interactions provide a crucial context for learning and development, especially during the preschool years (ages 3–5) when children experience rapid growth across all developmental domains. Guided by Vygotsky’s sociocultural theory, this qualitative, observation-based study explored prosocial and antisocial behaviors within the context of peer interactions. It included 13 preschool children from three age-specific classrooms (3-, 3.5-, and 4-year-olds) at a private childcare center in the United States. This configuration allowed for the examination of prosocial and antisocial behaviors across the 3- to 4-year-old continuum, with 3.5-year-olds included as an intermediate group. Eighteen 30-min observations were conducted to identify behavioral patterns, developmental trends, and contextual differences across five classroom activities. Thematic analysis revealed distinct behavioral patterns, with the three most frequently occurring types of prosocial behavior being (1) socially proactive, (2) socially responsive, and (3) sharing, while the two most commonly occurring types of antisocial behavior were (1) physically aggressive and (2) verbally aggressive. It also revealed developmental trends, with 4-year-olds exhibiting the highest percentage of overall prosocial behavior and the lowest percentage of overall antisocial behavior compared with their 3- and 3.5-year-old peers. Contextual differences were also evident: for overall prosocial behavior within each age group, unstructured playtime elicited the highest percentage among 3- and 3.5-year-olds, while circle time elicited the highest percentage for 4-year-olds. Unstructured playtime also accounted for the highest percentage of overall antisocial behavior across all age groups. All of these key findings were interpreted through the lens of self-regulation and theory of mind.</p>