We recently described a giant double stranded DNA virus called Mimivirus, isolated from amoebae, which might represent a new pneumonia-associated human pathogen. Its unique morphological and genomic characteristics allowed us to propose Mimivirus as a member of a new distinct Nucleocytoplasmic Large DNA viruses family, the Mimiviridae. Mimivirus-specific features, namely its size and its genomic complexity, ranged it between viruses and cellular organisms. This paper reviews our current knowledge on Mimivirus structure, life cycle and genome analysis and discusses its putative evolutionary origin in the tree of species of the three domains of life.