The mathematical modelling of the transport and transformation of trace species in the atmosphere is one of the scientific tools currently used to assess atmospheric chemistry, air quality, and climatic conditions. From the scientific but also from the management perspectives accurate inventories of emissions of the trace species at the appropriate spatial, temporal, and species resolution are required. The chapter has discussed bottom-up methodologies to estimate global and regional emISSIOns. These methodologies are based on activity data, emission factors (amount of emissions per unit...