Abstract: The GFDL CM2.1 global coupled GCM exhibits strong interdecadal and intercentennial modulation of its ENSO behavior.  To the extent that such modulation is realistic, it puts large error bars on ENSO metrics diagnosed from centennial and shorter records -- with important implications for model assessment and intercomparison, climate projections, and historical and paleo records.  We shall discuss the causes and effects of this ENSO modulation, including its links to the background climatology, inter-event memory, seasonal phase locking, and decadal predictability.