More than 80% of all lab-on-a-chip systems rely on optical detection. In most cases this is done by external bulk optical elements. We present an approach where advanced multimode optical elements are integrated with a microfluidic system. In order to ease integration of the optical circuitry, the waveguide height and width are adapted to the dimensions of the microfluidic channels. Typical dimensions for the multimode waveguides are 40 μm x 40 μm. The integrated optical elements include tapers, waveguide crossings, and spectrometers. The devices are designed, simulated and subsequently fabricated in polymer on a silicon substrate. A glass lid bonded to the polymer layer seals the microfluidic channels and provides a top cladding for the waveguide circuitry. Arrays of specially designed components are evaluated to extract precise basic parameters like coupling and propagation loss. To increase compactness of the waveguide circuitry waveguide crossings with different angles are evaluated. It is found the angles down to 25° between the crossing waveguides show little (< 0.25 dB) excess loss. Integrated spectrometers using a reflective, concave echelle grating are fabricated and evaluated. It is shown that spectral range, resolution and linear dispersion of such miniaturized devices can be adapted to the needs of micro total analysis systems (μTAS).© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.