Geochronology is the science of determining the ages of rocks on earth. In the GEOMAR Geochronology Laboratory the absolute ages of rock forming processes in marine and terrestrial environments (crystallization, cooling) are determined by 40Ar/39Ar laser dating. The 40Ar/39Ar method is based on the decay of the naturally radioactive Potassium isotope 40K, has evolved into the most commonly applied isotopic dating technique, and is applied to a wide range of studies, including the emplacement, cooling and thermal history of plutonic rocks, volcanic processes and hazards, the formation and evolution of the oceanic crust, metamorphic events and processes, plate tectonics and mantle evolution, sedimentation rates and processes, the ages of catastrophic events, bolide impacts and mass extinctions, the calibration of biologic and paleomagnetic time scales, and the history of early man.40Ar/39Ar dating involves the irradiation of samples with fast neutrons in a nuclear research reactor, fusion of rock or mineral samples under ultra high vacuum conditions with a high-powered continuous laser, and analysis of the released argon isotope abundances with a high-sensitivity mass spectrometer. GEOMAR Geochronology laboratory facilities include advanced sample preparation equipment, an MAP 216 noble gas mass spetrometer, an online gas extraction and purification system, and a 20-Watt Spectra Physics argon ion laser, permitting manual to fully automated single-crystal and step-heating analyses. High precision ages are determined, ranging from the Precambrian to the Holocene.