Is it illegal to go into an abandoned house?

It all really depends on two things:

How long it has been vacant, if the house has been empty for 12 years for example then it's probably safe to say that whoever owns it doesn't have imidiate intentions of selling it or fixing it up.

The law; if you intend to go sight seeing, make sure you aren't trespassing; if there are locks, fences, and or signs, these all mean you would he trespassing on the property.

Also it would be safe to ask the local police.