This chapter outlines modern applications for miniaturized gas-phase chemical instrumentation systems and provides several examples of microfabricated MEMS devices and data analysis methods presented in the literature. It introduces some of the challenges that portable systems face and identify a few future directions and emerging trends in the field. The chapter focuses on ion mobility devices, metal oxide–based sensors, and chemical analysis systems, including micro-gas chromatography technologies and e-nose devices. It argues that the machine learning, pattern recognition, and statistical learning methods will be used interchangeably. One of the most common types of miniature chemical sensors is the metal oxide gas sensor. In fact, metal oxide–based carbon monoxide and combustible gas sensors have been commercially available since the 1970s for use in automotive monitoring, air quality monitoring, and other toxic gas detection applications. At the core, metal oxide–based sensors rely on electrical impedance changing as a function of gas composition and concentration.