In most forms of interstitial lung disease (ILD), a multi-disciplinary discussion is of great benefit in clarifying the diagnosis. Histopathology forms the cornerstone of diagnosis, but accurate pattern identification on histopathology is not only observer-dependent but depends on the accuracy and adequacy of tissue sampling. This chapter adopts a systematic approach to histopathological and cytological diagnosis of the ILDs. Issues pertaining to BAL are also addressed: When does BAL sampling without biopsy suffice? In what context should interpretation of BAL cellularity be done? How may the dominant cell type on BAL influence the diagnostic approach to ILD? What special findings on BAL might lead to suspicion/diagnosis of a specific condition? What disorders may be ruled out with reasonable certainty based on BAL? The histopathologic features of several conditions that might suggest a specific diagnosis are systematically considered, with a separate discussion on granulomatous interstitial lung disease. A discussion on the choice of the modality for lung biopsy is also included.