It is a shield bug.
But which one...? There are lots of species worldwide.
Let me assume you are in the UK. There are fewrer but still quite a lot of shield bugs there, and each of them goes through several different stage as they develop - and those can look very different to the adults.
I'm going to need another opionion here, as I can't readily find a good match.
Perhaps the best is scarce tortoise shieldbug, Eurygaster maura (a late nymph thereof). But there are some details that don't match, especially the ridge down the middle of the back. Also, it is rare; it is confined to the south of England (Essex, Kent, Surrey and Hampshire).
Do you live in that area?
One of the best online references to British Bugs:
(there is a gallery of shield bugs here - http://www.britishbugs.org.uk/gallery/heteroptera/Pentatomoidea/pentatomoidea.html)
Like I wrote: other opinions welcome!
Assuming the ID is correct on the following page, I think it may be a hairy shieldbug (Dolycoris baccarum). The British Bugs site doesn't appear to have a mid to late instar photo but compare it with the bottom left image here:
I guess the proof in the pudding would be whether or not it was hairy at all - your photo is a bit too fuzzy (no pun intended) to tell.
That's a good call, thanks.
The distribution is less of an issue as well.