Reference Data Series No. 1 (RDS-1) is an annual publication - currently in its thirty second edition - containing estimates of energy, electricity and nuclear power trends up to the year 2050. RDS-1 starts with a summary of the situation of nuclear power in IAEA Member States as of the end of 2011. The data on nuclear power presented in Tables 1 and 2 are based on actual statistical data collected by the IAEA’s Power Reactor Information System (PRIS). However, energy and electricity data for 2011 are estimated, as the latest information available from the United Nations Department of Economic and Social Affairs is for 2009 only. Population data originate from the World Population Prospects (2010 revision), published by the Population Division of the United Nations Department of Economic and Social Affairs. The 2011 values again are estimates. As in previous editions, projections of future energy and electricity demand and the role of nuclear power are presented as low and high estimates encompassing the inherent uncertainties involved in projecting trends. The RDS-1 estimates should be viewed as very general growth trends whose validity must be constantly subjected to critical review.