By my reckoning (some are questionable), there have been **21 voluntary, 8.5 forced, 5 deaths and 4 unknowns** (Ace, Amy, Rory, River). If you consider death a forced exit, then it's around a third forced and two thirds voluntary (most often for love). ### 1st Doctor * Susan wants to leave (she falls in love), but decides that she can't (because she has to look after her grandfather). The Doctor forces her out because he knows that it's best. * Barbara and Ian were kidnapped by the Doctor. They leave voluntarily (the Doctor opposes it) when they find a way to get home. * Vicki falls in love and leaves voluntarily. * Steven leaves voluntarily in order to lead a peace effort. * Katarina dies (sacrifices herself to save the Doctor). * Sara Kingdom (dubiously a companion) dies by accident. * Dodo leaves voluntarily (with no real reason given). 5.5 voluntary exits, and 2 deaths. ### 2nd Doctor * Polly and Ben join the 1st Doctor by accident. They voluntarily leave when they have a chance to resume their normal lives. * Jamie and Zoe are forced to leave by the Time Lords. * Victoria leaves voluntarily to live a normal life (much the same as Martha) 3 voluntary exits, and 2 forced. ### 3rd Doctor * Brigadier Lethbridge-Stewart played a similar role to a companion, but the Doctor left him, so doesn't really count for this question. The same is true for Sergeant Benton and Mike Yates. * Liz Shaw never travelled in the TARDIS, but was essentially a companion. She voluntarily left UNIT (with no real reason given). * Jo leaves voluntarily (she falls in love). 2 voluntary exits. ### 4th Doctor * Sarah Jane Smith is forced to leave when the Doctor is summoned to Gallifrey. * Harry leaves voluntarily to resume his normal life. * Leela leaves voluntarily (she falls in love). * K-9 (Mk I) leaves voluntarily (to stay with Leela) and K-9 (Mk II) is given to Romana (he can only survive in E-Space). K-9 (Mk III) and K-9 (Mk IV) didn't travel with the Doctor. * Romana leaves somewhat voluntarily (she chooses between staying in E-Space and returning to Gallifrey, rather than either and staying with the Doctor). 4 voluntary exits, and 2 forced. ### 5th Doctor * Adric dies (saving Earth). * Nyssa leaves voluntarily, much like Steven. * Tegan gets left behind by mistake once (but rejoins the Doctor after an Earth year), but eventually leaves voluntarily, when she releases she cannot handle life with the Doctor. * Turlough leaves voluntarily (when he finds out his home planet has improved). * Kamelion is destroyed by the Doctor when he is again taken over by the Master. 3 voluntary exits, and 2 deaths. ### 6th Doctor * The Doctor is forced to abandon Peri, and he thinks that she has died (it's later revealed that she did not). 1 forced exit. ### 7th Doctor * Mel leaves voluntarily for another type of travel. * We don't know about Ace, because the series was cancelled before we found out. Apparently the intent was for her to leave (presumably voluntarily) to train to become a Time Lord. 1 voluntary exit. ### 8th Doctor * Grace declines to travel with the Doctor (a voluntary exit before anything even starts!), although it's questionable whether she can be considered a companion. 0.5 voluntary exits. ### 9th Doctor * Adam Mitchell was forced out by the Doctor after not even one episode. * Jack Harkness is left behind, although after *Utopia* he voluntarily returns to Torchwood. 2 forced exits. ### 10th Doctor * Rose is forced to leave (although she does actually stay with a Doctor). * Mickey Smith leaves voluntarily (multiple times!). * Donna is forced to leave because she cannot survive knowledge of the Doctor. * Martha leaves voluntarily to live a (somewhat) normal life. The 10th Doctor also had several 'one-off' companions - most lists include them as companions although they never travelled with the Doctor and only appear in a single episode. None of them can be considered to have "left" the Doctor, although two do die (Astrid Peth and Adelaide Brooke), one of whom probably would have travelled with the Doctor. 2 voluntary exits, 1.5 forced, and 1 death. ### 11th Doctor Amy and Rory are still travelling with the Doctor, so we don't know how they will leave yet. River is similar to a companion, but (as far as we know) doesn't travel with the Doctor (yet), so can't be said to have left - although it's likely that it will be a forced exit. (The Amy-Ganger was a companion of sorts, destroyed by the Doctor). The above only includes companions who appeared on TV episodes (and the 8th Doctor movie); those that appear only in spin-off media (audio dramas, novels, etc) are not included. The [TARDIS Index File][1] has more information about the companions, including the fate of these additional companions. [1]: http://tardis.wikia.com/wiki/Companion