This volume contains papers presented at Symposium H, "Molecular Electronics," held November 29-December 2 at the 1999 MRS Fall Meeting in Boston, Massachusetts. As Si-based microelectronics marches to ever-smaller feature sizes, the point is rapidly approaching when conventional MOSFETs will no longer operate. Such devices depend on the concept of "doped regions" which can be envisioned to be ever so small until you get to the point where there is only a handful of dopant atoms. Tunneling across ultra-thin films is another major issue. Single molecules as active device elements is a concept that goes back to the 1970's, but in recent years very substantial progress has been made in synthesizing molecules suitable for such purposes. Carbon nanotubes have been a prime candidate for both nanowires and active devices. Molecules as small as single benzene rings, and larger, more complex molecules have been synthesized and measured, showing reproducible current-voltage characteristics. Unusual features are large negative differential resistance and large shifts of resonant peaks as functions of temperature.