We review recent work on the theory of rapid, reversible bimolecular reactions in solution. Based on the relationship between density fluctuations and the radial distribution function, we discuss molecular and phenomenological definitions of rate coefficients for reactions of the form A + B ↔ C. We show that the pseudo-steady-state approximation to the radial distribution function yields the asymptotic formula obtained by Lee and Karplus (1987), Naumann (1991), and Szabo (1991), and leads to the relationship between the molecular and phenomenological rate coefficients noted by Garst (1989, 1991). This approximation is valid when the time scale for reaction is much longer than for diffusion. Close to equilibrium the pseudo-steady-state approximation can be shown to be compatible with the Onsager hypothesis for fluctuations (deGroot & Mazur 1962).