Integrated curriculum defies rigid definition but usually involves the purposeful combination of two or more subjects. Based on the degree of integration, models of implementation can be placed along a continuum ranging from fusion, to multidisciplinary, then interdisciplinary, and finally to transdisciplinary. Integrated curriculum emerged in the Progressive era of the early twentieth century and has continued with varying degrees of popularity into the early twenty-first century. Common characteristics are evident among all periods of implementation: a constructivist approach, relevance and engagement through student-centered experiential inquiry into real-world problems, a concern for the cognitive and affective domains, and flexibility in methods of implementation. Implementation of integrated curriculum may require challenging shifts in teacher practice and in school-based logistics. However, it offers positive trade-offs that motivate its adoption. One benefit is deeper, more authentic learning that effectively addresses the cross-disciplinary twenty-first-century skills. Regarding achievement, students in integrated programs perform as well as, or better than, their peers who have learned in traditional discipline-based programs. Additionally, integrated programs increase student engagement and cultivate positive attitudes and behaviors. For teachers, integration streamlines curriculum coverage and assessment while invigorating their professional commitment.