Inhaled drugs have been recognized for their great potential for improved drug delivery, but so far only a fraction of their potential is benefiting clinical practice. Many new promising drug candidates are in development pipelines and some of them are approaching processes of regulatory agencies. Overall, investigation in inhalation drug development is intense. Inhalation administration route provides benefits over other routes of administration, e.g. avoidance of first-pass-metabolism, and convenience for patients in administrations without repeated injections. However, drug development of inhaled substances is complicated and usually development times are longer than ordinary drug development. Additional investigational needs in development of inhaled drugs are dealing with formulation, devices and variability of respiratory function between human subjects. Radiopharmaceuticals provide invaluable tool to explore these issues non-invasively. First, radiopharmaceuticals can be used in vitro for proof of concept studies. Second, they are used to resolve depository issues, as well as to give timeframe for clearance of substances in target organs. Third, radiopharmaceuticals' new potential use is focused on extending non-invasive imaging technique towards true pharmacokinetic modelling via calculations of target organ follow-up and exposure. Exposure estimates of a particular drug candidate make safety evaluation feasible early in the drug development. Safety issues can be resolved by investigating exposures of target organs, and depository and absorption issues in airways.