Is IE8 backwards compatible?

Hopefully some has some knowledge of this. I'm fast realising that IE8 certainly can't be ignored anymore. I had previously put it down to either a) just a buggy piece of software, or b) still to small a piece of the market share. Now, however, I'm seeing more and more requirement to make sure sites work on it. My question is can I switch over to testing on IE8 only and safely 'believe' that the resulting code will still work on IE7? Any thoughts/knowledge would be helpful. Cheers.