Broncho-alveolar lavage is a simple investigation to carry out and enables the gathering of biological information such as inflammatory and immuno-competent cells, tumour cells, microorganisms as well as mineral particles which are found in the biological milieux present in the distal air spaces. The performance of LBA assumes a mastering of the correct technical aspects of this method of investigation, particularly in the phase of injecting and recovering of physiological saline and an understanding of the indications and contraindications of the technique. The interpretation of the information gathered by cytological, microbiological, mineralogical and biochemical study of the bronchoalveolar lavage liquid should comply with a strict set of rules and justifies a close working contact between the clinician and the biological scientist.