As others have said, having seen Doctor Who
is certainly not a prerequisite for watching Torchwood.
. That being said, as Guy Gardener
said, season 2 of nuWho
set up Torchwood
and the two-part finale introduced it, however it introduced a very different Torchwood
than you see on the show (which is explained on both shows).
The main character, Captain Jack Harkness, was introduced in season 1 of nuWho
and spent several episodes travelling with the Doctor. The end of Torchwood
's first season dovetails with the end of Doctor Who
season 3, though you can find the clips on YouTube, but they aren't necessary, if you're fine with a character disappearing and not knowing where they went.
A first season episode features a classic-Who
villain, which most Who
fans hated Torchwood's
take on. It's directly linked to the season 2 finale of Doctor Who
. A late season 2 episode fills you in on the characters and how they came together to work at Torchwood, which has a few nods to Who
, but nothing blatant.
In short, having seen Doctor Who
will fill in details, but it is not necessary. It compliments Torchwood
and may increase your enjoyment, in terms of understanding things, but Torchwood
is not that complex, that missing that knowledge will hurt your understanding or enjoyment of the show.