Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of lawsonite for water and element cycling in subduction zones, occurrences of fresh lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix lawsonite. As lawsonite blueschist is much more common than lawsonite eclogite, we survey the composition of lawsonite in representative localities of blueschist, including blueschist associated with eclogite (lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and lawsonite-rich veins. This dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones.