Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In the first part the authors discuss the process of positron moderation and technical aspects of positron beam production and application. The second part is (roughly) organized in sections that apply to increasing time scales appropriate to the positron-solid interaction. These are (a) first encounter and scattering effects, (b) energy loss and stopping profiles,

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Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers grown by molecular-beam epitaxy on Si (100) substrates. Defects at the initial growth interface and throughout the overlayer have been observed and depth profiled. In addition, field-driven positron drift observed in some of the epilayers is shown to be consistent with estimated concentrations of (active) interfacial impurities. The study demonstrates that positrons can be used nondestructively to profile structural defects and electric fileds in thin films and at interfaces.