When's best to prune a tree?

[Lakelord]

As per the thread title really. I have a Robinia tree in the garden that's got a bit too lush for my liking so needs sorting and trimming back.
I'm not one to interfere with nature at the best of times so is this OK to do at any time or should it be done during a particular season?

[Simpo Two]

I had a rampant Robinia/Acacia with enormous thorns on it. It kept the giraffes away (never saw one, ever) but was just too vigorous and the thorns made it hard to handle (see 'blood'). I had it pruned back a couple of times but it just came back bigger and faster. Eventually I got rid of it.

That said, the best time to prune trees is usually autumn when they've stopped growing.

[richyb]

Winter is the naturally dormancy period for most trees. I would do any major pruning them.