The 2009 pandemic influenza A (H1N1) was responsible for the first influenza pandemic of the 21st century. The virus- a previously unknown triple-reassortant virus containing segments of avian, human, and swine origins- generally caused mild disease. Unlike seasonal influenza, 2009 pandemic influenza A (H1N1) primarily affected adults 18 to 64 years of age. During the course of the pandemic, public health officials tried to facilitate diagnostic procedures and share information about treatment modalities globally. Efforts to contain the spread of 2009 pandemic influenza A (H1N1) included personal protective mechanisms and the 2009 H1N1 vaccine, which was not produced quickly enough or in large enough quantities. The lessons learned from this pandemic should be applied to ensure better preparedness in case of future pandemics.