Molecules adsorbed on rough metal surfaces exhibit enormously strong infrared absorption (10–103 times compared to the corresponding free molecules). The so-called surface-enhanced infrared absorption (SEIRA) is similar to surface-enhanced Raman scattering (SERS) in nature. However, there is a remarkable difference between SEIRAS and SERS regarding the metals available: SERS is observable only on coinage metals except for some specific cases, while many other metals including transition metals also are available for observing SEIRA. Owing to the versatility, SEIRA has a potential for wider applications than SERS. The utility of SEIRA has been demonstrated especially in in-situ monitoring of reactions at the electrochemical interface. Nevertheless, the mechanism of SEIRA is still not very clear. In this lecture, fundamental aspects and mechanistic models of SEIRA will be discussed.