This chapter is devoted to Lipps’s theory of science, that is, the question of how he conceived the foundations and organization of scientific knowledge. In the first section, I discuss how Lipps envisioned the nature and status of scientific disciplines between the years 1877–1880 and 1883, drawing a distinction between “the sciences of inner experience” and “the sciences of outer experience,” while in the second section I focus on the period 1886–1893, highlighting the “rationalistic turn” that occurred in his conception of the relationships between science and experience. The third section is devoted to an analysis of Psychologie, Wissenschaft und Leben (1901), in which he addresses the question of the nature and status of the Geisteswissenschaften. In the fourth section, I discuss the views that he expounds in “Inhalt und Gegenstand” (1905), namely, his fourfold taxonomic approach to scientific knowledge and his discussion of the place of causality, normativity, and phenomenality in sciences. Finally, in the fifth section, I comment on Lipps’s writings of the years 1906–1908 that marked the development of his late theory of science, focusing on his reflections on the foundations of natural science and natural-scientific concepts, in line with contemporary debates about conventionalism, fictionalism, vitalism, and energeticism.