I'm taking my adult grandsons to Paris in August. We want to stop over in London. How do I do that when making a reservation on-line?

Most airline booking websites allow you to specify a multi-city itinerary. I'd start with www.kayak.com to see what options are out there.

You have several options. Let's say you're originating in Chicago, you would enter

Flight 1. Chicago-London
Flight 2. London-Paris
Flight 3. Paris-Chicago

or

1. Chicago-London
2. Londo-Paris
3. Paris-London
4. London-Chicago

or, you might find it better to book two roundtrips

Chicago-London-Chicago
London-Paris-London

There are some risks when doing the latter (2 separate round trips), especially if you use different airlines. If the Paris-London flight is late and you miss your London-Chicago flight you may have to fork out the $ to buy a new flight home. You need to play around a little bit and see what options pop up.

Another option is to what's called an open jaw ticket.

Chicago - London
Paris - Chicago

you could then book a simple one way flight from London to Paris as a separate ticket.

I would book an open jaw ticket :

Chicago to London and Paris to Cicago

Then take the Eurostar to Paris - faster, more convenient (city cener to city cener) and more comfortable than flying. Just as with air tickets book in advance to get the best prices.

I'd definitely do as nytraveler suggests. Open jaw (multi-city) Chicago to London and Paris to Chicago. You don't want to fly from London to Paris if you are staying in London - getting out to Heathrow, advance check-in, travel in from CDG, etc all add to the time, hassle and expense.

But you can take the tube or a short cab ride from your hotel to St Pancras station, get on the Eurostar train, and be in central Paris before your plane takes off at LHR.

Be sure to book the Eurostar ahead of time since walk-up/last minute fares can be very high.

again, ditto on the Open-jaw ticket. That's what I do most of these days if I want to visit more than 1 city on my trip and those cities are not close to one another.

Most of the time, these open-jaw tickets cost the same as regular r/t from one city.

You can decide whether you want to fly into London first, or fly into Paris first.

Which city to fly into and fly out of kind of depends on which airline and which airport and what flight times there are available.

I don't know where your city of origin is, but if your home airport/airline offers the day flight to London, that's the one I would take, because I find the day flight much easier on my body in terms of jetlag than the overnight flight from N Am.

Yup, open-jaw tickets and take the Eurostar. Search online for airlines or consolidators that will allow you to get that Eurostar ticket at a discount when you book the flight. I think Air France often offers that option.

I booked an open jaw ticket through WWW.ATIflights.com and saved over $500 on the flight through Delta and my actual flights are through Delta. When you go on the website just click multicity and decide where you want to go. Good Luck