The evolution of structural highs bordering the seaward edges of major modern forearc basins is controlled by their location at inception and two primary processes--seaward accretion, and landward understuffing. Many highs probably initiate as crustal ruptures. Seaward accretion broadens the highs. Landward understuffing elevates the highs and tends to move them in a landward direction by tilting or by failure of their inner edges by compressional folding and faulting. Forearc basins behind the highs grow seaward if accretion is accompanied by submarine elevation of progressively younger accreted section added to the structural highs. Forearc basins become narrower, however, when the highs bordering their outer edges tilt landward or fail under compression in the absence f such contemporaneous accretionary growth. Accretionary broadening has previously been discussed and is exemplified by the postulated late Mesozoic history of the northern California forearc. Landward tilting, or compressional failure, is exemplified by the Cenozoic history of the northern California forearc and is suggested by a cross-section across the Guatemalan forearc, where small landward migration similar to that of highs bordering smaller Aleutian and Chilean forearc basins is recorded.