The drive to higher and higher lightwave transmission capacity has come in part through deregulation, but also from demand for internet access, telecommuting, and broadband services. Historically, in order to meet that demand, service providers used higher transmission rates, but now there is a clear trend toward wavelength division multiplexing (WDM), which has some significant advantages. When the switching is done in the optical layer, it will be possible to have true wavelength routing and a reconfigurable network. To implement this vision, a number of sophisticated optical components will be required. In the late 1980's, researchers started to explore whether the functionality of passive optical components, such as fibers and taps, could be combined, similar to the way that electrical components (resistors, capacitors, and transistors) are combined in an integrated circuit (IC). The primary goal was to reduce manufacturing costs, but the additional advantages of ICs, such as reduced size, increased performance (especially at high speeds), and greater reliability, are also be desirable in photonics. The silicon wafer can be used as a platform to attach lasers and detectors as well, hence the name "silicon optical bench" for this technology. Silicon optical bench technology leverages off the silicon IC industry for low cost, high quality flat substrates as well as processing developments and equipment. Initially, the focus was on passive waveguide structures and considerable effort went into designing a materials and processing technology that would be compatible with standard semiconductor manufacturing equipment and would also produce planar waveguides that were well-matched to standard single mode (SM) optical fiber. As the devices become more complex - from routers, to reconfigurable add/drop filters, to Er-doped planar waveguide amplifiers - new materials and processing challenges have arisen.