The group of λ Bootis stars

In this article the current status of the knowledge for the group of lambda Bootis stars is reviewed. These metal poor objects are quite outstanding compared to other chemically peculiar stars of the upper main sequence. Up to now no conclusive theory has been developed which is able to explain the majority of observational results. There is, however, an increasing interest in observational as well as theoretical results in order to develop and test astrophysical models such as accretion of circumstellar matter and diffusion processes in stellar atmospheres.