Leg 169 of the Ocean Drilling Program built upon the success of Leg 139 in investigating the geological, geophysical, geochemical, and biological processes at sediment-covered spreading centers in the northeast Pacific Ocean. The primary scientific objective of Leg 169 was to investigate the genesis of massive sulfide deposits by drilling two deposits at different stages of maturity: Middle Valley at the northern end of the Juan de Fuca Ridge and Escanaba Trough at the southern end of the Gorda Ridge. The four primary topics of investigation during this leg were (1) the mechanism of formation of massive sulfide deposits at sediment-covered ridges, (2) the tectonics of sedimented rifts and controls on fluid flow, (3) the sedimentation history and diagenesis at sedimented rifts, and (4) the extent and importance of microbial activity in these environments. Postcruise research of the shipboard scientific party submitted to the Scientific Results volume for Leg 169 includes investigations into porewater chemistry; sulfide and clay mineral petrology and geochemistry; the physical properties of basalt, sediments, and sulfides; the structural framework of the feeder zone of a seafloor hydrothermal system; and estimates of the biomass and microbiological diversity attending the growth and evolution of a sediment-hosted massive sulfide deposit. Manuscripts accepted for publication elsewhere at the time of submission of this synopsis include a study of deep-sea sedimentation resulting from cataclysmic floods off the west coast of North America, pore-water chemistry and systematics, and metal sources in the massive sulfides. Postcruise analyses of sulfide mineralization and wall rock alteration were still in progress at the time this overview was written, but several preliminary conclusions can be drawn from the available data. Mineralization at the Middle Valley site was the result of a structurally focused, long-lived hydrothermal system. This resulted in deposition of at least 100 m of massive sulfide at the Bent Hill deposit and the formation of three stratigraphically stacked massive sulfide lenses at the ODP Mound site. Both deposits are underlain by extensive Cu-rich feeder zone mineralization. A subseafloor silicification zone formed a caprock that forced hydrothermal fluid flow into permeable sandy horizons where high-grade copper ore formed by replacement of sediment. Fracturing of the silicified caprock near the time when the intrusion of basaltic magma uplifted Bent Hill may have been responsible for a second pulse of hydrothermal activity at the Bent Hill Massive Sulfide deposit. Drilling through the silicified zone reinitiated hydrothermal activity at both the Bent Hill and ODP Mound deposits. Drilling at Escanaba Trough revealed extremely high sedimentation rates related to breaching of glacial lakes in the Columbia River basin. The sheeted sill style of oceanic crustal development that characterizes the central rift in Middle Valley is generally lacking at Escanaba Trough. Sulfide deposition at Escanaba Trough occurred due to a recent hydrothermal pulse that led to widespread diffuse venting of high temperature hydrothermal fluids. The hydrothermal fluids reacted extensively with the sediments, enriching the massive sulfide deposits in sediment-derived metals. Pore fluids include both high-salinity brines and low-salinity fluids formed by phase separation. This short but intense hydrothermal episode appeared to lack strong structurally controlled fluid flow paths. Intense hydrothermal alteration and stringer zone mineralization were not encountered beneath massive sulfide at Escanaba Trough.