Two contrasting studies of metal and semiconductor interfaces are presented. The first study examines the coverage dependent structural transitions of Sn/Cu{100} examined by Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD). A model consistent with both the Sn surface coverage and the complex split beam LEED pattern observed is suggested for the low coverage (0sn = 0.21ML) ordered phase, based on a p(2x2) structure with light antiphase domain walls. For the higher coverage phases, rotated domain p(2x6) (0sn = 0.37ML) and p(3V2xV2)R45° (9sn = 0.50ML) structures are observed. Double scattering simulations based on c(2x2) local periodicity are presented for these phases. The possibility of surface alloy versus overlayer models is discussed. The monolayer phase (0sn = 0.625ML) appears to involve de-alloying of the c(2x2) mixed layer to form an ordered Sn overlayer above Cu{ 100}. In the second investigation the initial stages of formation of the Ge/GaAs(001) interface are studied by Normal Incidence X-Ray Standing Wave Spectroscopy (NIXSW) and Core Level Photoelectron Spectroscopy (PES). After submonolayer deposition of Ge onto the As-rich GaAs(001)-(2x4) surface and annealing to 875K, a sharp (1x2) LEED pattern is observed which is attributed to Ge-Ga dimerization along the [1 1 0 ] direction. This is explained by oudiffusion of the first layer arsenic atoms and the germanium dimerizing with the second layer gallium atoms. A model based on the formation o f Ge- Ga dimers is presented based on the NIXSW and PES results. As the thickness of the Ge overlayer is increased beyond 4ML, additional weak fractional-order spots from a (2x1) reconstruction appear. From about 6ML coverage an equal intensity double domain (lx2)+(2xl) pattern is observed which is attributed to Ge-Ge dimerization as found on elemental Ge(100).