Miniaturization of spectrometers for microfluidic applications is currently an intense area of research. This thesis is devoted to the design, fabrication, and testing of a planar optical microspectrometer. Optical microspectrometers are potentially important candidates for noninvasive detection and identification of cells and other biological material. The focus of this study was to improve the functionality of the present lab-on-a-chip devices, through monolithic integration of microfluidic channels, optical waveguides and diffraction gratings on a single disposable opto-bio-chip. One important device for spectral analysis of cells and other analytes is the diffraction grating and most fluorescence detection systems reported to date use off-chip bulk optical gratings. Here, an integrated approach based on a planar curved focusing transmission grating fabricated together with microfluidic channels, optical waveguides and a collimating lens in a single layer of polydimethylsiloxane (PDMS) is described. Layers of lower-index PDMS were bonded to this layer to provide optical and fluidic confinement. Because of the index contrast with the outer layers, light can be confined in the central “optofluidic” layer, so that propagation of light in the guiding (core) layer is governed by total internal reflection. The fabricated microspectrometer was tested using a variety of light sources including three different lasers and a broadband white light source. The optical performance of the fabricated microspectrometer closely matches the design specifications. In summary I have made the following contributions: i) developed a new optofluidic integration process in PDMS (Chapter 5). ii) developed a set of numerical tools for analyzing individual elements of a spectrometer such as planar gratings and lenses or the device as a whole (Chapter 3). iii) fabricated and tested a novel, curved focusing transmission grating, and explored its use for fluorescence spectroscopy (Chapter 6). iv) developed a novel dynamic strategy for sensing using the diffracted orders of the grating (Chapter 7).