Coupled ocean-atmosphere interactions in the tropical Pacific led to the development of an El Niņo of moderate intensity in 2002-03. This event, though not as strong as the 1997-98 El Niņo, nonetheless had significant impacts on patterns of weather variability worldwide. The evolution of the 2002-03 El Niņo will be described through comprehensive satellite and in situ observations from the El Niņo/Southern Oscillation (ENSO) Observing System. These observations underscore the importance of both episodic atmospheric forcing and large-scale low frequency ocean-atmosphere dynamics in the development of the event. The degree to which the observations are consistent with current theories of the ENSO cycle will be discussed. Implications for the predictability of ENSO will also be discussed by comparing forecast advisories issued during the event with its actual evolution.