Halogens have an established impact on atmospheric composition, but further quantification of their global air-quality and climatic impacts is needed. This thesis documents the development of a simulation to represent the atmospheric chemistry of iodine within a wider halogen framework. Following the development of the iodine simulation (Chap- ter II-III), a coupled model is presented (Chapter IV) that brings together and builds upon previous halogen (Cl,Br) studies in GEOS-Chem. Finally this model is used to investigate impacts of halogens on climate (Chapter V), and iodine sourced aerosol (Chapter VI). Significant implications of halogens on oxidants are shown. Iodine alone reduces the tropospheric O3 burden by ∼9 %, with “coupled” halogens (Cl,Br,I) reducing it by ∼15 %. Global mean OH concentrations decrease by 4.5 % on inclusion of halogens. However, this is due to competing factors. The O3 loss decreases primary production, whereas conversion of HO2 to OH via photolysis of hydrohalic acids tends to increase it. Chlorine provides a potent new oxidant in the model. For some VOCs (C2H6, (CH3)2CO) Cl oxidation provides up to ∼20 % of their sink. The effect of halogens on the tropospheric O3 radiative forcing (RFTO3) is investigated. Halogens cause a feedback effect, dampening the increase of tropospheric O3 between the pre-industrial and the present day by ∼20 %, therefore reducing RFTO3. Aerosol-phase iodine is also investigated and shown to regionally contribute up to 101 % of DMS sourced sulfate aerosol mass. In the pre-industrial, iodine aerosol can regionally contribute up to 21 % of the sulfate mass. Iodine and halogen chemistry in general are required to understand tropospheric composition and processes. Uncertainty in the emissions, chemistry and loss processes for halogens is high. Further in-situ observations and elucidation of key parameters in laboratories are urgently needed to refine our understanding of this important aspect of atmospheric chemistry.