The chapter is devoted to review the theories of spatial density fluctuation in aqueous solutions. The density fluctuation is characterized by the pair correlation function, which is essentially the variance of the probability distribution of density. In Sect. 2.1, we introduce a physical quantity referred to as “density field” which defines the density of liquids at a local point. In Sect. 2.2, we define the density pair correlation function by the variance of density fluctuation. Section 2.3 is concerned with the equation referred to as Ornstein–Zernike (OZ) equation which governs the density pair correlation function. Sections 2.4 to 2.6 are devoted to explain applications of the OZ equation to a class of liquids called “simple liquids” in which molecules are just spherical. Section 2.7 is concerned with the RISM theory. The RISM theory is an OZ equation generalized to so-called molecular liquids in which a molecule is not spherical but a composite of atoms. However, the theory is not applicable to water in which intermolecular interactions involve the electrostatic interaction. The RISM theory applicable to water, referred to as the XRISM theory, is explained in Sect. 2.8. The theory to explore the solvation structure of a biomolecule in water, referred to as the 3D-RISM theory, and its application to protein is reviewed in Sects. 2.8–2.11.