In April, 2009, CDC identified a novel influenza A virus detected from 2 children with febrile respiratory illness in southern California. The virus quickly emerged and spread globally and by 5 May, confirmed cases had been reported from 41 US states and 21 countries worldwide. Since the virus had never been identified before, little was known about the characteristics of the virus and how the pandemic would progress-would it be severe, how efficient would viral transmission be, would transmission be sustainable, what would the spectrum of illness, factors associated with severe disease, and causes of death be, and what risk groups would be most affected? Field investigations and epidemiologic studies in the United States and elsewhere were critical in helping answer these questions and characterizing the virus and the pandemic. This supplement will report results from field and epidemiologic investigations conducted in the United States since April 2009.