In a word - unrecognisable. It's up there with a Will Wheaton level of change. 

Chunk, or 'Jeff Cohen' to his friends, put in an appearance at a pre-Emmy party and set more than a few tongues wagging given no one recognised him. Well, at lot can happen in twenty years.

According to Us Weekly, since starring in the cult kids comedy back in 1985: "Cohen's days of searching for pirate treasure may be over, but his time in Hollywood isn't. Now an attorney, the child star graduated from UCLA School of Law in 2000, and co-founded the Beverly Hills entertainment law firm Cohen Gardner LLP in 2002. (He last acted in the TV movie Perfect Harmony in 1991.)"

For those of you don't watch The Big Bang Theory, this is what the very pretty kid from Stand By Me / Star Trek  looks like now.