The non-linear optical technique of Second Harmonic Generation (SHG) has been used to study static and dynamic interfaces. The polarisation dependence of the SHG intensity from the water/air interface is shown to be affected by very low levels of contaminant. A new method for calculating the non-linear surface susceptibilities is described which enables improved error estimation of the calculated quantities. SHG was then used to study molecules adsorbed to the water/dodecane interface. Paranitrophenol (PNP) was first used as a probe molecule and showed the interface concentration of PNP became saturated above bulk aqueous concentrations of 5 mM. This behaviour is successfully modelled by a Langmuir approximation with k_a/k_d = 23300 ± 2500 moldm⁻³. The polarisation dependence of the SHG signal was found to be invariant with interfacial concentration and represented an average molecular tilt angle of 60° from the surface normal. Addition of sodium chloride to a 3 mM aqueous PNP solution was found to increase the surface concentration of PNP at the water/air interface with no change in the polarisation dependence of SHG signal. Addition of tributylphosphate (TBP) to the water/PNP/dodecane system resulted in a slow interaction between TBP and PNP causing a reduction in SHG intensity with increasing bulk TBP and PNP concentration. Investigation using a novel flow cell technique enabled the kinetics of the interaction to be measured. The interaction was found to be first order with a time constant of 0.087 ± 0.012 s⁻¹. The dye molecule Rhodamine 6G (R6G) was also found to show Langmuir type adsorption to the water/dodecane interface with k_a/k_d = 21.5 ± 1.4 x 10⁶ M⁻¹. The polarisation dependence of the SHG signal was found to show a step change at surface coverages greater than 0.6 of a monolayer. At high laser power a photochemical reaction was observed with the SHG signal decreasing with time. This decay was shown to be first order with respect to R6G coverage at the interface.