By extensive and systematic molecular dynamics simulations eight stable cellulose crystal structures have been identified. mie organized them in a coherent scheme, which gives a clue to possible interconversions between the various structures. From this scheme it can be inferred that mercerized cellulose II could in fact be a parallel structure or contain parallel regions. This parallel cellulose II structure fits into the same unit cell dimensions as antiparallel cellulose II and therefore can remain unnoticed. Regenerated cellulose II is likely to have all hydroxymethyl groups in the gt conformation similar to what was found in two recent determinations of the cellotetraose structures. Due to its characteristic layer structure cellulose I could crystallize in several different packing modes. Biosynthesis, however, enforces a parallel packing. It is shown that I alpha and I beta have a similar and specific parallel packing mode.