The planet is currently going through a period of global climate change, the pace of which is unprecedented in geological history. Marine phytoplankton contribute approximately 50% of the total global primary productivity and play a vital role in global carbon cycling. Consequently it is extremely important to understand the impact that global climate change will have on the ecological performance of these organisms. In this review we summarise current understanding of the influence that global climate change has on the physiological properties, productivity and assemblage composition of marine phytoplankton. While most phytoplankton are likely to show little direct effect of elevated CO₂ on photosynthetic rates, some, including the ecologically important coccolithophorids, are likely to show significant stimulation of growth. The rise in temperature consequent upon the elevated atmospheric levels of CO₂ and other greenhouse gases will stimulate growth of some species and increase ocean temperatures in some areas beyond their current optima. However, more importantly, increasing global temperatures will stimulate stratification of the water column which, in tropical and mid-latitudes will exacerbate nutrient limitation in surface waters. This in turn will lead to changes in phytoplankton assemblage composition, primary productivity and sensitivity to UVB radiation.