Community-acquired pneumonia (CAP) is a common condition affecting about 1/1,000 of the adult population per year. It occurs when bacteria enter the alveolar spaces of the lung initiating an inflammatory response which leads to the clinical features of cough, sputum production, breathlessness and sometimes chest pain and haemoptysis. At the end of the last century the causal relationship between bacteria and pneumonia was established and many of the early discoveries about the causes of CAP were made in Europe. Some 41 different prospective studies have established that approximately 10 different microbial pathogens regularly cause CAP with occasional cases due to other rarer causes. The frequency of these organisms in Europe is similar in most countries, but there are some geographic differences. Differences in frequency are also apparent according to illness severity. It is generally recognised that Streptococcus pneumoniae is the most important causal bacterium in all countries. A relatively recent development has been the appearance and spread, in some of the common causative bacteria, of resistance to commonly used antibiotics to which they were once sensitive. The frequency of such resistance does vary markedly between European countries. However, published data is often difficult to interpret. The reasons for this are that the frequency of resistance varies according to the age of the patient, the site of the sample, the clinical diagnosis, the use of prior antibiotics and the influence of special groups e.g. those with cystic fibrosis. The impact of in vitro antibiotic resistance on clinical outcome is still poorly understood, but recent studies are helping to clarify this issue and will be discussed.