Theophylline reduces asthmatic bronchial obstruction; this desired therapeutic action but also the occurrence of unpleasant and dangerous side effects depends directly on its tissue and serum concentration. An effective therapy therefore requires repeated determinations of this serum concentration. Safe dosage is complicated by complex pharmacokinetics such as considerable interindividual differences in distribution space and hepatic clearance. Children eliminate theophylline faster than adults; thus they require larger doses and demonstrate higher fluctuations of their serum levels. Various exo- and endogenous factors can alter the clearance rate and therefore have to be considered. Intravenous infusions of theophylline are the mainstay in the treatment of status asthmaticus. A flow chart of therapeutic actions is presented and supportive intensive care is described. In prophylactic oral therapy slow-release preparations are preferred. Optimal long-term medication is best initiated via stepwise increments of the daily dosage plus one or more final adjustments after the determination of the drug's serum concentration.