We classify Veech groups of tame non-compact flat surfaces. In particular we prove that all countable subgroups of GL + (2, R) avoiding the set of mappings of norm less than 1 appear as Veech groups of tame non-compact flat surfaces which are Loch Ness monsters. Conversely, a Veech group of any tame flat surface is either countable, or one of three specific types. * Partially supported by MNiSW grant N201 012 32/0718 and the Foundation for Polish Science.† Partially supported by Landesstiftung Baden-Württemberg.‡ Partially supported by Sonderforschungsbereich/Transregio 45 and ANR Symplexe.