The group of lambda 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 c ompared to other chemically peculiar stars of the upper main sequence. Up t o now no conclusive theory has been developed which is able to explain the majority of observational results. There is, however, an increasing interes t in observational as well as theoretical results in order to develop and t est astrophysical models such as accretion of circumstellar matter and diff usion processes in stellar atmospheres.