Constructivism is a learning theory where students actively construct knowledge through experiences and mental representations. Key figures include Piaget who developed cognitive stages of learning, Bruner who viewed learning as active, and Vygotsky who emphasized social learning. According to constructivism, students learn by analyzing problems and building on prior knowledge, rather than through direct instruction. Implications for the classroom include experiential, problem-based, and student-directed learning with assessments like journals, portfolios, and observations.