This volume covers Leg 66 of the cruises of the Drilling Vessel Glomar Challenger. Plate tectonics has raised a number of new and intriguing questions, especially concerning subduction complexes, subduction mechanics, and evolution of subduction zones. Answering these questions has been difficult, owing in part to the problem of studying active subduction complexes. Seismic reflection investigations have been frustrated by the complex structures of subduction complexes, and Deep Sea Drilling Project efforts to sample subduction complexes have been similarly frustrated by thick hemipelagic slope apron deposits that limit penetration into the deformed rocks and sediments. Leg 66 was part of a broader effort to overcome these difficulties and contribute to the resolution of critical questions about subduction. Leg 66 is actually part of two broader investigations of subduction complexes and convergent margin geology—one led by the Active Margin Panel (AMP) of the International Phase of Ocean Drilling (IPOD) to drill selected convergent margins to elucidate structure and evolution and the other, led by the University of Texas Marine Science Institute Galveston Geophysical Laboratory to investigate the tectonics of the Middle America Trench from the Cocos Fracture Zone to the Riviera Fracture Zone. The synergism of these two efforts resulted in the drilling of Legs 66 and 67 off southwestern Mexico and Guatemala, respectively. The present volume and Volume 67 are thus companion efforts. Leg 66 began in Mazatlán, Mexico in March of 1979 and ended in Manzanillo, Mexico in May of 1979.