The beta-lactams are mainly active on the bacterial cell wall. Although it was long claimed that their clinical efficacy was due to the bactericidal effect observed in vitro, it is now demonstrated that this effect cannot be the sole explanation for the therapeutic successes noted with these antibiotics in clinical situations. Several effects other than bactericidal are observed at subinhibitory concentrations and allow a better understanding of the activity of the beta-lactams. These are, among others, inhibition of bacterial growth, the post-antibiotic effect, the morphological effect, loss of virulent factors and decrease of adherence. --Following recent developments in the field of the beta-lactams, the new antibiotics exhibit a major broadening of their microbiological spectrum that in some cases allows simplification of antibiotic therapy: in some clinical situations monotherapy can be used instead of a combination. Some antibiotics have very good pharmacokinetic properties, allowing once-a-day dosage and outpatient treatment. --Finally, adverse drug effects such as neutropenia, hitherto considered infrequent, have been recognized more often. Explanation of their physiopathological mechanisms could favour the development of less toxic antibiotics.