Invasive fungal infections, predominantly aspergillosis and candidiasis, are the most important causes of morbidity and mortality in immuno-compromised patients. Antimycotics are used for the treatment and prophylaxis of fungal infections. Antifungal drugs show large intra- and interindividual variabilities in pharmacokinetics. The accurate measurement of the individual drug concentration in serum/plasma is essential for therapeutic control and successful results.