How long has your dog been throwing up for? Was there a change in diet? Is your dog eating or drinking reguarily? Is your dog urinating and having bowel movements? You should consult your vet. Could you supply more information on your dog's current condition?

It could be not serious (just stomach upset like people get, or he ate something outside, etc), and it could be very serious (twisted bowel, common in dogs). Get to the vet.

We really need more information. There are so many things that could cause a dog to throw up. This could be minor or extreme.

We need to know the overall health of your dog. Is he lethargic, is he eating, is he drinking, is he otherwise okay? How old is your dog, is he up to date on his shots? So many things we need to know.

When in doubt your vet is your best bet. But coming here with a one line question won't give you many answers. We simply can't tell you based on the info you provided.