Volatile organic compounds (VOCs) from biogenic and anthropogenic sources are important constituents of the atmosphere with effects on air quality and climate. Current uncertainties in measurements and models relate to their roles in tropospheric ozone and secondary organic aerosol formation, yet there have been few measurements of their fluxes from contrasting chemical environments. Additional measurements with greater spatial and temporal resolutions are required to constrain uncertainties in atmospheric chemistry and climate models. This thesis presents long-term measurements of VOC fluxes and concentrations in two contrasting environments: central London and the Brazilian Amazon. VOC concentrations were quantified by proton transfer reaction-mass spectrometry and fluxes were calculated using the virtual disjunct eddy covariance method over a period of several months at sites in central London and the Amazon rainforest. In central London, traffic was found to be the main source of aromatic compounds. Oxygenated compounds and isoprene showed strong correlations with light and temperature, suggesting biogenic, evaporative, or secondary atmospheric origins. The seven VOCs measured in central London had a five-month average total emission rate of 1.4 mg m-2 h-1. Comparisons with local and national emission inventories showed that modelled emissions were largely underestimated. Measurements of isoprene and monoterpenes at the remote ZF2 site in the Amazon rainforest showed an 11-month average total emission rate of 2.7 mg m-2 h-1 with considerable seasonal variation, which could not be accurately reproduced using the light and temperature based MEGAN algorithms. This thesis presents the first long-term VOC flux measurements providing information at high temporal resolutions on seasonal variability at an urban site and a pristine tropical forest site. They confirm that the Amazon rainforest is an extremely strong source of reactive carbon to the Earth’s atmosphere exceeding emissions from a developed megacity, such as London, per unit area.