The report focuses on the physical, chemical and biological processes and factors involved in the absorption of reactive gases. Emphasis is placed on the importance of these factors in developing dosimetry models, special consideration being given to the role of lung fluids and tissues. Several dosimetry models are discussed and illustrations of predicted results presented to demonstrate the application of the models to the uptake of NO2 and O3, and to demonstrate the use of models in determining the effects of physical, chemical and biological parameters on dosimetry predictions. Gaps in our knowledge and understanding of the processes of dosimetry are pointed out and research recommendations made to increase our understanding of the processes and to enhance the development of dosimetry models.