The trivia section on the Doctor Who IMDB page tells us that the only reason we have 'Next Time' trailers is because during production of the first Series of the reboot the show consistently ran under its 45 minute length. But after Eight years, (almost) Seven series and a couple of specials, the idea of Doctor Who without a Next time trailer seems positivley ridiculous. A good 'Next Time' trailer does what all good trailers do. It needs to strike a balance between intriguing the audience, and not giving too much away. What sets the 'Next Time' trailer apart is that it comes hot off the heels of a brand new episode, and so if the episode itself has been particularly good, the short preview at the end can tease the building of threads that have been woven into the episode (See almost any trailer relating to Season 5 and the 'Cracks in time' arc, or nearly any RTD/Moffat 'event' episode). Some may avoid them completely because they want to go in spoiler free, the 'Next Time' trailer from the perspective of the production team has become the show's most important ways of asking viewers to come back next week. I have two honorable mentions to make, that didn't make the list for different reasons. The first is the teaser/lack of teaser for Series 4's 'Journey's End'. Cast your minds back to 2008 and remember the surprise of a potential David Tennant regeneration, and the credits roll, but instead of a teaser we get the words 'To be continued...'. Now, apart from the resolution being predictably underwhelming, it still deserves mention just because of the amount of anticipation it managed to build up by using this technique. (This is also seen in some of the other series finales) Secondly, I'd like to mention the latest 'Next Time' trailer, the one for 'The Name of the Doctor', because damn does it seem intriguing. Or rather, it would if the preceeding Series had made any effort to build up the mysteries so that the answers we are being teased with in the finale held any kind of weight. And with that, we begin our countdown...