Archaea is one of the three domains of life and studies of archaeal biology are important for understanding of life in extreme environments, fundamental biogeochemical processes, the origin of life, the eukaryotic cell and their own, unique biology. This thesis presents four studies of the archaeal cell, using the extremophilic Sulfolobus and ocean living Nitrosopumilus as model systems. Cell division in crenarchaea is shown to be carried out by a previously unknown system named Cdv (cell division). The system shares homology with the eukaryotic ESCRT-III system which is used for membrane reorganization during vesicle formation, viral release and cytokinesis. Organisms of the phylum Thaumarchaeota also use the Cdv system, despite also carrying genes for the euryarchaeal and bacterial cell division system FtsZ. The thaumarchaeal cell cycle is demonstrated to be dominated by the prereplicative and replicative stage, in contrasts to the crenarchaeal cell cycle where the cell at the majority of the time resides in the postreplicative stage. The replication rate is remarkably low and closer to what is measured for eukaryotes than other archaea. The gene organization of Sulfolobus is significantly associated with the three origins of replication. The surrounding regions are dense with genes of high importance for the organisms such as highly transcribed genes, genes with known function in fundamental cellular processes and conserved archaeal genes. The overall gene density is elevated and transposons are underrepresented. The archaeal virus SIRV2 displays a lytic life style where the host cell at the final stage of infection is disrupted for release of new virus particles. The remarkable pyramid-like structure VAP (virus associated pyramids), that is formed independently of the virus particle, is used for cell lysis. The research presented in this thesis describes unique features of the archaeal cell and influences our understanding of the entire tree of life.