Reflectance anisotropy spectroscopy (RAS) has proved itself to be extremely sensitive to both surface reconstruction and ultrathin coverages of material on semiconductor surfaces. This in situ technique therefore lends itself to monitoring the formation of low-dimensional systems in a wide range of growth environments. The following systems have been studied under molecular beam epitaxy (MBE) conditions: (i) the deposition of sub-monolayer coverages of Si, to form Si delta-layers within GaAs; (ii) the As/P exchange reaction which leads to the formation of InAs surface quantum wells on the InP(001) surface; (iii) the self-organised growth of InAs quantum dots via strain relaxation of ultrathin layers (< 2 ML) of InAs deposited on the GaAs(001) surface; (iv) the kinetics of hydrogen desorption from vicinal Si(001) surfaces. In our studies of Si/GaAs, it has been possible to deduce both that RAS is sensitive to coverages greater than or equal to 0.005 ML Si/GaAs, and to estimate the level of activity of Si donors within a single delta-plane. Analysis of time-resolved RAS measurements has led to: an activation energy of 1.23 +/- 0.05 eV being determined for the As/P exchange reaction on InP(001); a detailed study of the conditions which influence InAs island formation on GaAs; and the suggestion of a new desorption pathway for H on vicinal Si(001) surfaces.