In an annual summary of the climate of 2003, the Climate Monitoring Branch of the National Climatic Data Center provides an overview of conditions throughout the US and around the world during the past year. As the scientific community works to better understand our changing climate, continual monitoring provides vital information on climate variability, continuing trends and the incidence of extreme events. Our primary focus is on placing the recent climate record in historical perspective based on more than 100 years of temperature and precipitation measurements. Included is a discussion of century-scale variability and trends in temperature and precipitation as well as extreme events such as droughts, tornadoes and hurricanes. The increase in global temperatures recorded over the past 100+ years has occurred in all seasons and throughout almost all regions of the world. The significance of this rise is supported by the absence of a similar increase at any time during the past 1000 years. There is also evidence that temperatures are now rising at a rate much higher than the average rate of the past century. Evidence of significant trends in extreme events is less clear. For example, while recent Atlantic basin hurricane seasons have been some of the most active on record, no discernable long-term increase in hurricane frequency has been observed.