Foreign body infections by coagulase negative Staphylococci are an important and growing problem in our hospitals. Only recently have we started to get some data on the specific virulence factors that permit the otherwise non-pathogenic Coagulase Negative Staphylococci (CNS) to be so successful in causing foreign body infections. Adherence of the Coagulase Negative Staphylococci to the foreign body is a first and crucial step. Several genes and gene-products have been identified that enhance staphylococcal adherence to biomaterials. Adherence is followed by accumulation; in this phase the Coagulase negative Staphylococci organise themselves into a complex multilayer of cells covered with polysaccharide. This we call the biofilm. Finally coagulase negative Staphylococci undergo complex and as yet non-defined metabolic changes that in combination with biofilm formation allow them to persist on the foreign body and become less susceptible to antibiotics. Few data are available on the factors involved in the accumulation and persistence phase.