Method and apparatus for external cavity laser absorption spectroscopy There is disclosed an apparatus, and corresponding methods, for determining one or more characteristics of a sample in an absorption cell using laser absorption spectroscopy. For example, the characteristic may be concentration of a species in the sample. The apparatus comprises an external cavity semiconductor laser comprising a semiconductor gain medium within an optical resonator. The absorption cell is located within the optical resonator of the external cavity semiconductor laser so as to be optically coupled with the gain medium. A controller is arranged to provide a varied injection current to the semiconductor gain medium. A photodetector is arranged to detect laser light output by the external cavity semiconductor laser. An analyzer is arranged to determine one or more characteristics of the sample from behavior of the detected laser light output as a function of the varied injection current.