The objective of this chapter is to introduce the bronchoalveolar lavage. Bronchoalveolar lavage (BAL) is a mildly invasive technique used to obtain secretions and cells for diagnostic purposes from the peripheral airways and alveoli. Bronchoalveolar lavage (BAL) has been reported to be a useful tool in addition to transbronchial biopsy for determining the etiology of pulmonary infiltrates, consolidation or solid lesions. Equipment required for bronchoalveolar lavage are flexible bronchoscope, sterile collection trap, suction tubing, sterile physiological saline solution, vacuum source, syringe, optional three-way stopcock and local anesthetic. The bronchoalveolar lavage BAL procedure is generally considered to be quite safe, and it has been used widely for research purposes and clinical evaluation of patients with a diverse spectrum of disorders that ranges from asthma and chronic obstructive pulmonary disease (COPD) to ILD, infection, and lung transplantation. Cytological examination from BAL, microbiology from BAL and evolving and future uses of BAL for patient management and clinical research are also covered in this chapter.