Weeds are major threat to agriculture and biodiversity as they out-compete crops and native species and contribute to land degradation. Changes in geographic distributions, abundances and life-cycles of weeds are the likely outcome of the effect of climate change. Natural evolution and certain specific characteristics such as short life cycles, dispersal mechanisms, may give the weeds a competitive advantage over less aggressive species under changing climate. Climate change may favour certain native plants to such an extent that they then become weeds. The dynamics of competition between weed and crop plants are affected by environmental conditions, and have been shown to change with atmospheric CO2 concentration, temperature, precipitation and adaphic factors. Invasive weeds like Lantana and Parthenium may become more aggressive under climate change especially due to increases in atmospheric CO2. Growth at elevated CO2 would result in anatomical, morphological and physiological changes that could influence herbicidal uptake rates, besides translocation and overall effectiveness. The physiological plasticity of weeds and their greater intraspecific genetic variation compared with most crops could provide weeds with a competitive advantage in a changing environment. There is a possibility that agricultural weed populations will evolve new traits in response to emerging climate and non-climate selection pressures.