Polymer-based microfluidic devices can offer a number of advantages over conventional devices, and have found many applications in chemical and biological analysis. In order to fully develop a lab-on-chip (LOC) device, the functional components, such as sensors and actuators, tend to be assembled to complete a functional device. But the integration of silicon chips into polymer-based microfluidic systems remains a virtually unexplored area. In this work, a novel silicon-in-plastic microfabrication technology is developed, which involves seamlessly integrating individual microfabricated silicon chips into a larger polymer substrate, where the silicon components provide functionality, and the plastic substrate provides system-level fluid handling. This technology employs low-cost polymer substrates and simple polymer processing techniques which are amenable to mass production. The fabrication and testing of two polymer microfluidic systems using the silicon-in-plastic technology are presented in this dissertation. The first integrated microsystem is a water-based chemical monitoring system based on microhotplate gas sensor and polymer microfluidics. The chemical monitoring system is designed to sample a water source, extract solvent present within the aqueous sample into the vapor phase, and direct the solvent vapor past the integrated gas sensor for analysis. Design, fabrication, and characterization of a prototype system are described, and results from illustrative measurements performed using methanol, toluene, and 1,2-dichloroethane in water are presented. The second one is an integrated UV absorbance detection system that uses silicon-in-plastic technology to seamlessly integrate bare photodiode chips into a polymer microfluidic system. Detection platforms fabricated using this approach exhibit excellent detection limits down to 1.5 x 10 8 M for bovine serum albumin (BSA) as a model protein. In addition to providing high sensitivity, sub-nanoliter detection volumes are enabled by the use of direct photodetector integration. The fabrication methodology is detailed, and system performance metrics including minimum detection limit, detection volume, dynamic range, and linearity are reported.