Return flight from Greece

BOTH

To add more detail to Lee's reply, Amsterdam will be your point of leaving the Schengen zone, so you will go through passport control. Customs is usually a non-issue at this point, but if you are getting any VAT refunds on goods purchased in the EU, you will do it here, so allow time for this.

Upon arriving at your first airport in the US, all passengers (whether terminating at that airport or connecting to other flights) must deplane, go through immigration (passport control), then pick up all checked luggage, then go through customs. If this is your final airport, you leave. If not, you re-deposit any checked bags at the "Baggage Recheck" desk just outside the customs door, then head to your next flight, passing through security on the way (since leaving immigration and customs puts you out of the secure area).

Don't worry, you don't have to remember these details; you won't be allowed to skip any of the required steps.

Thanks Harold. I guess my main question is will the hour and a half layover in Amsterdam be enough time to get through passport control etc?

Yes, it should be fine. Schiphol Airport allows transfers of 50 minutes, I believe, and I once made it in 45 (because of a schedule change, running, and with help from the transfer desk). If your connecting flights are both booked on one ticket and there's any serious delay in the first flight, then the airline will be responsible to put you on the next available flight. If your first leg were on an unaffiliated budget carrier (not recommended), you would need to build in more time. Note that the security process at Schiphol involves answering about 20 questions, so don't dawdle, even if you have more than an hour.