Foreword: 1st Alexander von Humboldt International Conference

The variability in the equatorial Tropical Pacific is characterized by sea-surface temperature anomalies and associated changes in the atmospheric circulation. Through an enormous monitoring effort over the last decades, the relevant time scales and spatial patterns are fairly well-documented. In the meantime, a hierarchy of models has been developed to understand the physics of this phenomenon and to make predictions of future variability. In this short review, I try to summarize theories and mechanisms about variability in such a way that these are accessible to a diverse group of researchers, such as that present in Guayaquil (in May 2005) at the First International Alexander Von Humboldt Conference "The El Ni o Phenomenon and its Global Impact".