Mountain bike chain?

My chain keeps only half engaging with the gears and slipping on the back wheel. I have replaced the rear gears and the chain, but still have the problem. Yesterday the new chain snapped and now i haven't got a clue what I can do to sort the bike out.
Ideas welcome Trev

I have also adjusted the gears correctly

If you've replaced those parts and the derailleur cable is truly adjusted correctly, then you need to check a couple other things or have a shop check them.

The derailleur itself should be set up properly.....limit screws, B-screw adjustment. Shimano and Sram both have .pdf instructions on their websites.

The first thing I thought of is that your derailleur hanger is bent. You'll need a $50 tool to check and adjust this, so unless you want to buy that, take it to a shop.

Proper chain length is important, too, but unless you cut it extremely short to where it magnifies any problem with rear adjustments, that shouldn't cause it.

If none of that solves it, then start looking at the freehub like someone else mentioned.

Sounds like the chain is too long.It could be the back changer needs adjusting.

Get professional help! This sounds like a compatibility problem, or something that Y! answers will not solve. Dangerous too!

Take it to a bike shop, either your chain is the wrong length and/or the rear derailleur is out of adjustment.

there is only two reasons why gears slip. the gears need tuneing or you have the wrong cain,cassette or crank. they must be the same make an modle to work correctly.

As all the others have said - if you dont knoe what you are doing take it to a shop or buy a decent maintanence guide. There are just too many variables in addition to the worn or stretched chain and badly adjusted mechs, you could also have worn teeth on the chain ring or even worn pawls on the freehub. Get proper help. Faliure off road can be serious