Until the early 90's, life on earth could be divided into two main domains: the Eukaryotes and the Bacteria. The separation between these domains was mainly based on differences in cell structure and molecular organization. In 1990 however, researchers isolated some organisms that, based on this classification, could not be placed in either domain. These organisms displayed some similarities to the bacterial domain (unicellular lifestyle, molecular organization), but also to the eukaryal domain (transcription and replication machinery). According to these findings, the existence of a new domain was proposed: the Archaea. Now, about a decade later, many other members of the Archaea have been isolated and they generally are regarded as a separate domain. Most isolates of the Archaea thrive at relatively extreme conditions, like high temperature, high acidicity or alkalinity, high concentration of toxic compounds, high pressure or high salt concentrations. These properties, in combination with their unique evolutionary position, draw a lot of attention towards the Archaea, both from the academic as well as the industrial world. In my project, I try to gain insight to the versatile lifestyle of the Archaea using a so-called functional genomics approach. Functional Genomics is a rapidly evolving field in which several research disciplines are integrated in order to couple genomic information to function. In these kind of projects, you will often find a combination of computer based methods ( in silico analysis) in combination with experiments that are performed in the lab in order to verify the in silico findings. Using the completed genome sequences that are available for several, both phylogenetically and physiologically diverse Archaea (today: about 14 genomes), I try to gain insight several aspects of the Archaea. For example: Metabolism: What kind of central metabolic pathways are present in the Archaea based on their genomes? How is their metabolism regulated? How did these pathways evolve and do we find these pathways also in other Prokaryotes and or Eukaryotes? Lifestyle: Why do the Archaea often thrive in extreme conditions? How did they adapt to these conditions and what mechanisms do they use to protect themselves against these conditions? Evolution: Based on their molecular organization, the Archaea have a distinct evolutionary position. Is this also reflected based on their genome contents? And how did the Archaea evolve with respect to the Bacteria and the Eukarya? Based on these questions, several research lines have been set out and some interesting hypotheses are being tested in the laboratory.