This volume summarises the outcome of the 13^th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP) on if, and if so under what conditions phytoplankton assemblages reach equilibrium in natural environments. Quite a number of ecological concepts use terms such as: ecological equilibrium, stability, steady-state, climax, stable state, etc. However, these ecological concepts often have been "translations" of scientific theories developed in physics or chemistry but they almost always lack scientific corroboration, the problem being that often these concepts remain vague and they are not formally defined. Here an attempt to formally recognize what "equilibrium" is in phytoplankton ecology is traced. The book also contains papers by leading scientists on the taxonomy of two selected key groups: cryptomonads and filamentous cyanoprokaryotes. This volume is addressed to all those involved in phytoplankton taxonomy and ecology and in ecology itself.