Systemic fungal infections are an important cause of morbidity and mortality among immunocompromised patients. New antifungal agents, such as triazoles, are now available, and the place of in vitro tests has to be discussed. It has been shown that interlaboratory reproducibility of in vitro susceptibility tests against fungi was low, due to the lack of standardization. Recently, the NCCLS defined conditions permitting a good interlaboratory reproducibility. However, the predictive value of in vitro susceptibility tests on the therapeutic outcome remain to be demonstrated, and is now under investigation. At the present time, susceptibility testing can be useful: in patients treated by amphotericin B for a severe fungal infection and who do not improve under therapy; to detect resistance to 5-fluorocytosine; to compare the sensitivity to triazoles before and after treatment, in case of therapeutic failure. Serum levels monitoring is useful to prevent the toxicity due to 5-fluorocytosine and to control the digestive absorption of triazoles, especially the lipophilic compound itraconazole.