Dense accumulations of certain phytoplankton make the ocean appear reddish. Some of these “red tides” poison marine life and negatively impact coastal fisheries and human health. Complex variability in coastal waters coupled with rudimentary understanding of phytoplankton ecology challenge our ability to understand and predict red tides. During fall 2002, multi-scale physical and biological observations were made preceding and during a red tide bloom in Monterey Bay, California. These intensive observations provided insight into the physical oceanography underlying the event. The bloom was preceded by intrusion of a warm, chlorophyll-poor filament of the California Current, suddenly changing physical and biological conditions through most of the bay. Enhancement of vertical density stratification followed the intrusion and created conditions favoring dinoflagellates. Favorable environmental conditions led to red tide inception in the northern bay, and advection strongly influenced spread of the bloom throughout the bay and out into the adjacent sea. Concentration of dinoflagellates in convergence zones was indicated by the development of dense red tide patches in fronts and in wavelike aggregations having the same scale as internal waves that propagated through the bloom.