Conventional semiconductor laser diodes have found limited applications in the areas of spectroscopy and sensors due to their multimode output and relatively large linewidth. Although single mode operation is possible with the use of distributed feedback techniques this tends to be at the expense of tunability. However operation of a laser diode in an external cavity gives a tunable narrow-linewidth source ideal for spectroscopic and sensing applications. This work details the design and construction of a tunable extemal-cavity semiconductor diode laser. The external reflector used is a diffraction grating operating in the Littrow geometry. The collimated first order diffracted beam is focused on the antireflection coated facet of a British Telecom Research Laboratory 1 3fjm laser chip 55cm away. The cavity has been characterised using a lm focal length grating monochromator and a Fourier transform infrared spectrometer. The laser behaved in the predicted manner exhibiting a greatly reduced lasmg threshold current and single mode operation. The device showed a grating tunable range of 37nm with intermode tuning, achieved by current change, of lOGHz/mA. A device linewidth of 3 3kHz was calculated and was found to be in good agreement with both calculated and measured results for similar configurations.