Lung deposition of aerosol is measured by a variety of methods. Total lung deposition can be measured by monitoring inhaled and exhaled aerosols in situ by laser photometry or by collecting the aerosols on filters. The measurements can be performed accurately for stable monodisperse aerosols. However, for polydisperse and/or unstable hygroscopic aerosols, measurement methods are limited and often unreliable. Regional deposition is assessed primarily by gamma scintigraphy. The scintigraphic method is useful for a general description of deposition patterns, but may not be adequate for quantitative regional dose estimation. New emerging three-dimensional lung imaging methods such as SPECT and PET, and innovative bolus aerosol delivery methods, have great potential for measuring detailed regional deposition. An accurate assessment of regional lung dose is difficult to achieve by the current methods. A multi-prong approach may be needed, with various methods and techniques, both in theory and experiment, to measure site-specific deposition dose.