An overview is presented of research at Cornell University on reliability-based design (RBD) of foundations for transmission line structures (TLS). Under sponsorship of the Electric Power Research Institute (EPRI), Empire State Electric Energy Research Corporation (ESEERCO), and others, a long-term research effort was conducted to develop a comprehensive RBD methodology for TLS foundations in both soil and rock under uplift, compression, and lateral/moment loading. The results of these studies are described herein in conceptual form, illustrating the basic concepts involved and key issues of geotechnical variability and uncertainty. Many are described in more detail in the geotechnical literature, so references are provided to the pertinent works. These very useful tools are readily available to the TLS design community. Copyright ASCE 2007.