Two major explanatory theories of Physical Chemistry, quantum theory and statistical theory, are studied in equal parts in this subject: (i) An introduction to spectroscopy and quantum chemistry, and their use in determining the energies, structures, dimensions and other properties of individual molecules. (ii) An introduction to the behaviour of assemblies of molecules using statistical mechanics applied to fluids to explain structure from intermolecular forces and thermodynamic properties. The laboratory work includes spectroscopic experiments for the determination of molecular energies and dimensions, comparing results with theoretical calculations.