The first half decade of the 21^st century has brought with it infectious outbreaks such as severe acute respiratory syndrome (SARS) [1], bioterrorism attacks with anthrax [2], and the spread of H5N1 influenza A in birds across Asia and Europe [3, 4] sparking concerns reminiscent of the days of the Black Plague. These events, in the context of an instantaneous global-media world, have placed an unprecedented emphasis on preparing for a human influenza pandemic [5, 6]. Although some argue that the media have exaggerated the threat, the warnings of an impending pandemic are not without foundation given the history of past influenza pandemics [7], incidence of H5N1 infections among humans [8], and the potential impact of a pandemic. Reports of the 1918 pandemic vary, but most suggested that approximately one third of the world’s population was infected with 50 to 100 million deaths [9]. Computer modeling of a moderate pandemic, less severe then in 1918, in the province of Ontario, Canada predicts 73,252 admissions of influenza patients to hospitals over a 6-week period utilizing 72% of the hospital capacity, 171% of intensive care unit (ICU) capacity, and 118% of current ventilator capacity. Pandemic modeling by the Australian and New Zealand Intensive Care Society also showed that critical care resources would be overwhelmed by even a moderate pandemic [10]. This chapter will provide intensivists with a review of the basic scientific and clinical aspects of influenza as well as an introduction to pandemic preparedness.