Specific texts on infectious diseases and those in textbooks on pulmonary pathology commonly approach the subject of infection in the lung from an etiological microbiological perspective, listing infectious agents by taxonomy followed by associated pathology. However, in practice one is faced with a specimen in which a reaction pattern is seen (usually of inflammatory nature) which may be indicative of a specific type of infection. In these circumstances an approach leading from reaction pattern to specific microbiological diagnosis is required. This approach will form the basis of this review. General aspects are covered without exhaustive discussion of specific microbiology. For more detailed discussion of specific entities the reader is referred to the excellent textbooks on pulmonary pathology and those on pathology of infectious diseases. The reaction patterns described here are combinations of gross pathology and cellular reactions. The breakdown of these patterns is somewhat arbitrary and artificial. In many cases the pattern will not be absolutely typical and many overlapping features will be present in individual cases.