We plan to develop statistical mechanical theory for the thermal and colligative properties of amphiphilic aggregates including monolayers, bilayer membranes, vesicles, and micelles. We have previously developed theory for the hydrocarbon chain configurations in these systems; predictions are in good agreement with NMR, neutron scattering, and other experiments. The fundamental conclusion from the "configurational" theory is that amphiphilic aggregates do not have the molecular organization of bulk phases of matter; they resemble interfacial phases. The proposed "thermal" and "colligative" theories for these semi-ordered "interphases" will be based on novel lattice methods and on adaptation of other reliable statistical mechanical techniques. There are three initial goals: i) to predict equations of state, as measured through pressure-area isotherms of monolayers, ii) to make quantitative the "principle of opposing forces", from which critical micelle concentrations and sizes and shapes of amphiphilic aggregates can be predicted, and iii) to develop a "regular solution theory" for the interfacial equivalents of bulk colligative laws, with the aim of predicting solubilities, vertical gradients, and lateral phase separations fo solute molecules within interphases, and solute-induced area changes such as those of cholesterol "condensation" or anesthetic drug "expansion". This work should have major biomedical impact inasmuch as it bears directly on the fundamental physical chemistry of lipid bilayer and biological membranes, on mechanisms of anesthetic drug action, and on fusion and permeability processes in membranes.