Although experiences of positive psychological states such as self-actualization, love, happiness, meaning, authenticity, and spiritual connection are of interest to a number of psychologists, they are also difficult to study with rigorous scientific methods. This article argues that a complementary approach, or methodological pluralism, is appropriate and adequate for such study. The underpinnings of two approaches to psychological research methods, natural science and human science, are reviewed. Although these two approaches typically lead to different research strategies, quantitative and qualitative methods respectively, they have common values in assessing the truth-value of research claims, consistency of research findings, and neutrality of scientific conclusions. Three examples of research using complementary approaches are given: life satisfaction, meditation, and nature-based peak experiences.