Van der Waals proposed his equation of state for denser fluids, (P+a/V²)(V−b)=RT in his 1873 dissertation, "On the Continuity of Gaseous and Liquid States of Matter." He tackled the problem anew by considering the two effects of molecules occupying space and of attracting each other. What van der Waals did in 1873 was to fit both gaseous and liquid states with one equation of state, and it was meaningful enough to make him one of the pioneers in theoretical studies of gases and liquids. His ideas being clearer than those of his predecessors, his theory gave a satisfactory molecular explanation for the then-known phenomena observed in vapors and liquids by many experimenters. The purpose of this paper is to show what he did in his life's work in the context of his days along with its historical background, and to investigate the emerging interconnections of his theory with other disciplinary topics such as kinetic theory and statistical thermodynamics, and finally to describe value and influence of van der Waals' work which inspired his successors to do both experimental and theoretical work.