Aerosol therapy has significantly improved the treatment of a variety of respiratory diseases. Besides the treatment of respiratory diseases there is currently also a great interest to use the lungs as a portal to introduce drugs for systemic therapy. The success of therapy with the application of aerosolized medicaments depends on the possibility to deliver the proper amount of drug to the appropriate sites in the respiratory system, thus limiting the side effects to a minimum. Aerosolized delivery of drugs to the lung is optimized if, for a given chemical composition of a medicine, the target of deposition and the required mass of drug to be deposited are precisely defined. The next step is the specification of the number of respirable particles or droplets, to be generated by appropriate devices. Another very important factor for successful aerosol therapy is the condition of the patient coupled with his or her inhalation technique.