Within a remarkably short period of time it became clear that TT viruses are widely spread globally, infect a large proportion of all human populations studied thus far and represent an extremely heterogeneous group of viruses, now labelled as Anelloviruses. TT virus-like infections have also been noted in various animal species. The classification of this virus group turns out to be difficult, their DNA contains between 2200 and 3800 nucleotides, related so-called TT-mini-viruses and a substantial proportion of intragenomic recombinants further complicate attempts to combine these viruses into a unifying phylogenetic concept. Although studied in many laboratories, the medically most interesting question of their possible pathogenic role in humans remains unanswered until today. It is not unreasonable to suspect that persistence of at least some specific TT virus genotypes may result in some infected individuals in a definable pathogenicity. We believe that this volume provides first hints in support of this view.