He's 20months and say's nothing.
Ok I'm lying slightly, he will say mama and dada but only if you ask him to and even then only when he feels like it. Think he's said each one twice, maybe 3 times.
He said NO to me Quite clearly a few months ago, nothing since.
If he see's something new or something he likes he will say "Oh wow". This has been the only consistent thing in his vocab and he's been saying it for months.
And that's it. He is great at instruction, go get your pj's/a nappy/your milk, close/open the door, brush your teeth, put your dirty socks in your wash basket
etc so he obviously understands a lot.
I wasn't too worried before, but just read another thread on MN with a couple of people saying 18months is very late for talking. 
So when do I start worrying?