During the late Paleocene-early Eocene (60-50 Ma), earth experienced the warmest conditions of the Cenozoic. Especially middle and high-latitude temperatures were much higher than today. Superimposed on this setting, a transient warming event (hyperthermal), known as the Paleocene-Eocene thermal maximum (PETM; 55 Ma), left a major mark on biogeosphere development. During a 170 k.y long interval, deep-sea and sea-surface temperatures rose by 5-10 C, leading to Arctic temperatures of ~23oC (by Tex86 method). Simultaneously, the global carbon cycle was severely perturbed, possibly through the massive release of methane trapped in deep-sea sediments. The deep-sea record shows that this climatic anomaly is associated with changing oceanic circulation and oligotrophy in the open ocean. Microbiota responded in various ways: deepsea and deep-shelf benthic foraminifera suffered major extinction, while other protists thrived, diversified or developed structural pathologies. While model studies are currently unable to explain PETM climate on a solid physical basis, the primary cause of the PETM also remains under debate. It has been variously attributed to volcanism, Atlantic continental margin failure, astronomical configuration, opening of the north Atlantic, or comet impact. Although its origin remains under intense debate, the PETM provides an intriguing natural experiment of rapid global warming and carbon-cycle perturbation, possibly with rates comparable to todays. Whereas most PETM research has been devoted to deep-sea records, epicontinental shelf sequences have provided key information on devastating eutrophy and anoxia in marginal basins of the Tethys replacement of coralgal reef systems by larger-foraminifera dominated ramp systems and evidence of eustatic sea-level fluctuations (20 30 m). The latter is unexpected since the early Paleogene greenhouse earth is generally considered to have been free of an Antarctic ice cap, although similar indications for polar ice caps were suggested for the late Cretaceous greenhouse. These data show that combined research on outcrops and deep-sea cores is a prerequisite to obtain a complete understanding of the earth system during the PETM, each archive contributing specific insights into a highly complex and dynamic system.