In TDM PONs, or simply PONs, the transmission capacity of optical fibers remains under-utilized with single-wavelength transmissions for both upstream and downstream traffic. Use of WDM transmission in PONs can significantly enhance the overall network capacity and coverage area. WDM transmission in PONs can be realized in several ways, such as by using WDM exclusively or combining WDM with TDM, leading to WDM PONs and TDM-WDM PONs (TWDM PONs). This chapter gives several candidate physical configurations to realize WDM and TWDM PONs, and describes some useful dynamic wavelength-bandwidth allocation (DWBA) schemes for the latter. We also briefly discuss the needs of open access to the PON-based access networks for the various stakeholders, such as service providers and network providers. Finally, we describe briefly the roles of optical networking in the access segment of mobile networks. (135 words)