This chapter are dedicated to theory of histopathology of respiratory system. Coloured diagrams and pictures of histopathology of respiratory system are given in detail. A brief write up about contents of chapter is given as introduction in the beginning of the chapter. Normal histology of respiratory system is described initially to provide the basis for better understanding of histopathology. Histopathology of Sinonasal inflammation, nasopharyngeal tumors, acute respiratory distress syndrome, pulmonary oedema, bronchopulmonary infections, asthma, hypersensitive pneumonitis, interstitial lung disease, emphysema, chronic bronchitis, pulmonary neoplasms is described in detail with the help of 36 well illustrated diagrams.