This thesis is concerned with some experimental and theoretical studies of weakly bound complexes. These complexes have binding energies of the order of a few kilojoules per mole, and consequently do not behave like normal chemical substances. The KrHCl complex and its isotopic forms, and NeDCl and its isotopic forms are studied in detail in the microwave and radiofrequency regions by Molecular Beam Electric Resonance (MBER) spectroscopy. The NeDCl complex is the first neon complex to be studied by microwave spectroscopy. Several new spectroscopic constants are identified from the experimental data of both systems, and all spectral features down to the resolution of the apparatus (about 1.5 kHz) have been explained. A model for the large amplitude vibrations that these complexes exhibit is proposed. Equilibrium structures and vibrationally averaged structures are subsequently derived for them. A more thorough analysis of atom-diatomic molecule interactions is made by deriving potential energy surfaces from the experimental data for the complexes. This method is analogous to the Born-Oppenheimer separation method for electronic and vibrational motion in molecules. The new experimental parameters, particularly DQ, the centrifugal distortion of the nuclear quadrupole coupling constant for the chlorine nucleus, help to fix the amount of coupling between the bending and stretching motions of the complex on the potential energy surface. The limitations of these derived surfaces are also examined. The method is extended to diatomic molecule - diatomic molecule complexes, and a potential energy surface for one of these complexes, (HF)₂, is derived. Refocussing studies on polyatomic complexes such as (CO₂)₂ are also presented, and the relationship between proposed gas phase structures and those of molecular crystals is discussed.