2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/112760We give some general properties of good and bad reduction, and some recent examples (worked out with Dipendra Prasad) of varieties having bad reduction not accounted for by their cohomology. We include some consequences of our remarks for varieties over number fields having good reduction everywhere.Text of a talk at Madras, August 2005History and Overviewtext