Recent developments in the use of diode lasers as ultra sensitive sensors are presented. Particular emphasis is placed on higher harmonic detection using the wavelength modulation technique. Experimental results are presented illustrating that this method, which uses a compact, portable apparatus, yields measurements with very high precision. Theoretical analyses of these experimental results are also provided. Novel ways of extracting detailed information of the probed sample are described. These include measurements of fine features, as well as simultaneous measurements of multiple overlapping lines. Such fine features allow one to accurately probe the collision dynamics of gaseous samples yielding values of temperature, density, and pressure. The method described here utilizes several novel features of higher harmonic detection. These features include a set of self-consistent measurements, by employing coherent detection up to the eighth harmonic order. The highest harmonic detection order achievable with this technique is limited by the bandwidth of the synchronous phase-sensitive demodulation apparatus and the noise figure for the detection system. Applications in basic science, in pollution monitoring and detection, and in industrial environments are briefly discussed.