Should I be riding on these???

I haven't checked my tires in a LONG time and I should of, but anyways I glanced at them today and DAMN. My back 2 have good tread and seem fine, but my front 2 tires have NO tread!!! The tires aren't that old, maybe have 10 to 20k miles on them, but why could the tread be that bad for only the front tires??? And should I continue driving like this, because there is No tread, the front ones look like they're about to give in. HELP!!!