Consideration of different potential emissions futures is essential for understanding and preparing for future climate change. This chapter briefly reviews the contribution of socioeconomic scenario uncertainty to overall climate change uncertainty. It then addresses the key drivers of emissions of greenhouse gases and other pollutants and how uncertainty in those drivers affects projections of future emissions. It then reviews the history of emissions and concentration scenarios and various approaches that have been taken, including the traditional scenario development approach of the Intergovernmental Panel on Climate Change (IPCC) as well as formal uncertainty analysis techniques and methods that can connect probability distributions back to individual scenarios of interest. The chapter closes with lessons learned from more than 40 years of socioeconomic effort to create scenarios to support climate change research.