Methanogens belong to the kingdom of Euryarchaeota in the domain of Archaea. The Archaea differ from Bacteria in many aspects important to molecular work. Among these are cell wall composition, their sensitivity to antibiotics, their translation and transcription machinery, and their very strict demands to anaerobic culture conditions. These differences may, at least partly, be responsible for the delay in availability of genetic research tools for methanogens. At present, however, the research within genetics of methanogens and their gene regulation and expression is in rapid progress. Two complete methanogenic genomes have been sequenced and published and more are underway. Besides, sequences are known from a multitude of individual genes from methanogens. Standard methods for simple DNA and RNA work can normally be employed, but permeabilization of the cell wall may demand special procedures. Efficient genetic manipulation systems, including shuttle and integration vector systems, have appeared for mesophilic, but not for thermophilic species within the last few years and will have a major impact on future investigations of methanogenic molecular biology.