Eurostar from London to Paris

Best way to travel between the two cities. Be sure to check-in at least 30 minutes before departure. St. Pancras is also a great place hang out. Lots of great places to eat, and shop ....so you may want to get there even earlier;)

Book your tickets well in advance (up to 120 days allowed) at www.eurostar.com to get discount fare tickets as low as €42.50. Last-minute tickets are very expensive (€155.00 or higher). You can print your own tickets. As Michael said, you are required to check in at least 30 minutes before departure time and you will have to go through security.

I love the Eurostar!

Try to get to your coach (the coach and seat number will be on your ticket) ahead of the hoard because the luggage space at the end of the coach fills up very quickly and then you have the problem of where to stash your bag(s).

Buy a carnet for the metro in the waiting area before departing for Paris.

And pack a picnic! I sin and get stuff from Harrods to eat and drink on the train. Sorta like a really expensive lunch on the fly. No one will look twice.

We were really excited to take the Eurostar under the channel and followed RS advice to visit Paris for a day during a UK/Ireland trip and did his one day tour. My husband thought it was the biggest non-event going under the channel. I think he was expecting an announcement saying "you are now under the English Channel" or the the lights to flash, or big signs along the tunnel....But nothing. But it was quick and comfortable. We left our luggage in the hotel in London and traveled with just a small daypack for the 24 hours in Paris.

Also Gare du Nord is often over-run with beggars nowadays: if anybody mutters "Speak English?" at you ignore them.

We've done it, loved it and wish all travel was this way--it is smooth, fast, efficient and allows you to party hardy, (if you so wish). Plus you can get up and walk around, or just admire the scenery as you travel through the French countryside.

Do not underestimate the need for advance check-in: you WILL be barred from boarding, particularly in London, if you don't arrive 30-min in advance at the check-in counter. . Think of it like airline checkin: don't play with the clock. . There is security control and immigration control, but you do both French and British controls at the departure stations - no controls on arrival. .
Don't expect anything about the tunnel crossing. It is just a long bore like many others, trains are pressurized, so you barely feel the pressure differential.

I second the recommendation to buy Paris Metro tickets from the desk in the waiting area in St Pancras in London. Very convenient.