A microscopic theory of orientational and structural instabilities associated with Langmuir monolayer phases is developed based upon methods of solid-state analysis. Surface harmonics are used to describe molecular orientational fluctuations providing a concise relation to the symmetries of the net. An explicit treatment of the translational-rotational coupling permits calculation of indirect molecular rotational coupling that is shown to lead to orientational and structural instabilities that are related to the nature of the intermolecular interactions and manifested by net symmetry, tilt, etc. Transitions from the highest symmetry "superliquid" phase are shown to be driven by an elastic instability which induces lattice strain. A mechanism for coupling the lattice strain to molecular tilt is developed. These results permit a consistent explanation of the formation of the Langmuir monolayer phases of n-alkanoic acids. It is demonstrated that some of the phase transitions may be viewed as ferroelastic. The thermoelastic properties of Langmuir monolayers are calculated and the free energy is derived as a function of orientational order parameters.