ABCs is an ongoing surveillance program that began in 1997. ABCs reports describe the ABCs case definition and the specific methodology used to calculate rates and estimated numbers in the United States for each bacterium by year. The methods, surveillance areas, and laboratory isolate collection areas have changed over time. Additionally, the way missing data are taken into account changed in 2010. It went from distributing unknown values based on known values of cases by site to use of multiple imputation using a sequential regression imputation method. Given these changes over time, trends should be interpreted with caution.