Climate change is a serious threat to Earth's ecosystems. Increased greenhouse gases and the associated rise in temperature have been implicated in melting glaciers, rising sea levels, acidification of marine and freshwater systems, increased ultraviolet-B radiation and increased fire frequency. The past century has seen a nearly 1 °C rise in global average temperature¹, with up to 7 °C of warming predicted by 2100 (ref. 2). Global warming has already led to significant shifts in the distribution, phenology and behaviour of organisms^{3, 4, 5}. Of 1,700 plant, insect, amphibian and bird species examined in a review by Parmesan and Yohe⁵, 80% had a poleward range shift of 6.1 km per decade, and 87% had an advancement in the timing of phenological events, such as breeding or flowering, of 2.3 days per decade. Such changes in distribution and phenology in response to climate change have received much attention in the literature, but only recently have studies begun to address the effects of climate change on development and growth. For many organisms, development and growth, and thus organism size, are affected by temperature and water availability^{6, 7, 8}. The increased temperatures and variability of precipitation associated with climate change are likely to influence the size of organisms, from primary producers to top predators^{9, 10, 11}. However, as we discuss below, the degree to which organism size is affected by temperature or precipitation variability is likely to vary within and between taxa, which could disrupt ecosystem functioning. Here, we briefly summarize the changes in organism size that are most likely a result of climate change and increased carbon dioxide levels, and theorize on reasons for the observed patterns of size declines. We present evidence from fossil records, experimental and geographic comparisons, and recent studies implicating current climate change in the shrinking size of organisms. We discuss the mechanisms that are most likely to be contributing to the observed patterns, exceptions to these trends and implications for biodiversity. Ultimately, our opinion is that this effect will become much more pervasive, and that research should focus on quantifying size trends more broadly, and identifying proximate and ultimate drivers of size declines.