In the pursuit of a portable gas detector/analyser we studied the components of an ion mobility spectrometer (IMS), which is a device that lends itself well to miniaturisation. The component we focused on was the ionizer. We fabricated a series of micro ionizers with micro electromechanical systems (MEMS) technology, which had a gap spacing between 1 and 50 μm and a thickness from 0.3 to 50 μm. They were used to examine micro discharges as such and as a means of ionization. In our measurements of electrical breakdown in small gaps we confirmed the deviation from Paschen's law for breakdown voltages in gaps below 5 μm. One important result is the identification of conditions for stable DC glow discharge in micro gaps. With planar electrodes we observed stable glow for factors of pressure times gap distance pd up to 0.2 Pa×m in N2, and up to 0.14 Pa×m in Ar. With thick electrodes the glow range was extended: up to 4 Pa×m in Ar, and 10 Pa×m in laboratory air at atmospheric pressure. The advantage of using discharges in micro gaps as the ionization principle is the low voltage and power that is necessary to drive a discharge. A prerequisite for using an ionizer in an ion mobility spectrometer is the possibility to operate such an ionizer at high, up to atmospheric, pressure. Our final micro discharge devices were operated in laboratory air for several hours without significant deterioration. A miniature ion mobility spectrometer was set up, in which miniature and micro discharge ionizers were applied as the ion sources. We extracted ions from micro ionizers and measured mobility spectra of gases and mixtures of gases (air, N2, Ar). The measured peaks in the mobility spectra varied depending on the gas. In the conclusions we suggest improvements that should increase the resolution and stability of our ion mobility spectrometer, so it may become useful for gas detection. The most important improvements will be a better control of the measurement conditions and of the initial extension of the ion pulse. In addition to our experimental results we present in this work an overview of the research that has already been done in our area of interest. As the basis of our research, the involved physical theory has been worked out. Consequently, the first chapters contain a compilation of relevant subjects, from the basics of electrostatics to the theory of DC glow discharge as far as we believe it can serve the reader in understanding our results.