The guidelines are to be used to guide the management of adults with lower respiratory tract infection (LRTI). As will be seen in the following text, this diagnosis, and the other clinical syndromes within this grouping, can be difficult to make accurately. In the absence of agreed definitions of these syndromes these guidelines are to be used when, in the opinion of a clinician, an LRTI syndrome is present. The following are put forward as def-initions to guide the clinician, but it will be seen in the ensuingtext that some of these labels will always be inaccurate. These definitions are pragmatic and based on a synthesis of available studies. They are primarily meant to be simple to apply in clinical practice, and this might be at the expense of scientific accuracy. These definitions are not mutually exclusive, with lower respiratory tract infection being an umbrella term that includes all others, which can also be used for cases that cannot be classified into one of the other groups. No new evidence has been identified that would lead to a change in the clinical definitions,which are therefore unchanged from the 2005 publication. Clin Microbiol Infect 2011;17(Suppl. 6): 1–24 The full version of these guidelines can be found on Wiley Online Library.