The original Star Trek series is now available in three box sets, one for each of the show's three seasons. The box sets follow the show's original air date order and feature bonus materials.
Originally, the series was released as 40 individual DVD packages with each disc in the package containing two episodes. This is a good way to buy Star Trek on DVD only if you want certain episodes, or would like them compiled in production order. There are, however, no real extras other than episode previews.