Buying tickets for Colosseum and Sistine Chapel in Rome online might be very fustrating. You have an option of buying the tickets at the gate, but it is subject to availability and you might have to queue for some time. It is a good idea to pre-book the tickets in advance and plan your visit to these popular tourist destinations.
Colosseum

The official website for buying ticket to Colosseum in Rome is this one:
https://colosseo.it/en/opening-times-and-tickets/
However, at the time of writing this post, the website was unavailable (for entire day!)
The adult tickets cost from €18 (normal entrance) to €24 (full experience with underground, which I recommend!). Pay attention that those tickets normally include Roman Forum, and Palatine Hills already! The ticket is normally valid for each of the included site 24h from the first entry.
The tickets go on sale 30 days before the date and tickets become available at the time of entrance (Rome time!) and some tickets gets added during the day (but that’s harder to spot). So you should try to book 30 days before, but give a try of navigating a page earlier to familiarize with it as you will have limited time to purchase the tickets (around 15min) and the tickets go away in a minute or two one added, but once you add the tickets to your cart they will be reserved for another 15min. Also have in mind that it seems the page has a limit of max 5 tickets, so if your group or family is bigger, that might be a problem.
You can buy audio guide at Colosseum when purchasing ticket for an additional price.
You can also buy the ticket on the spot in the booths next to Colosseum or even better you can buy them at Roman Forum or Palatine Hills with significantly smaller queue and use it to enter Colosseum. However, if you do want to see underground level you must pre-book it as it won’t be available on the day.
DO NOT BUY TICKETS FROM THE STREET! THIS IS A KNOWN SCAM!
The other option is to buy the tickets from re-sellers, who probably books those tickets from original website using automated bots, but do it at your own risk. At least make sure to purchase with your credit card, so you can get your money back from your bank is something goes very wrong (you can ask for chargeback if the service you’ve paid for was not provided, just make sure to gather all proof and documentation). BOOK FROM THESE RE-SELLERS AT YOUR OWN RISK!
getyourguide.co.uk website offers options from €35. Make sure to read small print, what is included! Limited to max 80 tickets (which makes me doubt if they can actually obtain the tickets). It has options with downloadable audio guide too.
viator.com website offer options from €18.90. Also read small print! Limited to max 4 tickets.
booking.com offers options from €29. Limited to max 4 tickets.
il-coloseo.it offers options fromo €28. Limited to max 3 tickets. Downloadable audio guide inicluded.
Vatican Museums and Sistine Chapel

Information about Sistine Chapel: https://www.museivaticani.va/content/museivaticani/en/collezioni/musei/cappella-sistina/storia-cappella-sistina.html
Official ticket site: https://tickets.museivaticani.va/home/calendar/visit/Biglietti-Musei
Vatican Museums and Sistine Chappel tickets can be purchased 60 days in advance. The same drill as with Coloseum tickets are made available at the exact time (in Rome time) when is the entrance time. You can definitely book 6 tickets, but buying more might be cumbersome.
Update 27 June 2024: I’ve tried to purchase the tickets exactly 2 months in advance and the tickets were released after mid-day. There were plenty of times available (some were already sold out!), but after entering the data (make sure you put names in correct order) and navigating to payment screeen I’ve canceled it and navigated back to check the data and surprise surprise my IP was blocked by Vatican (pope blocked me 😀 ) from any further purchases! Whenever I entered data and tried to make a purchase I was getting P1 error (500 error in form their payment service) on the screen. I’ve managed to buy only by another person buying it in their name from a different location. This must be some bot prevention mechanism.
The tickets do sell out quickly due to the resellers, so plan 2 months in advance and practice purchasing/navigating website in advance too. DO NOT BUY TICKETS FROM THE STREET! THIS IS A KNOWN SCAM!
BOOK FROM THESE RE-SELLERS AT YOUR OWN RISK!
getyourguide.co.uk offers prices from €31. Options with downloadable audio guide. Limited to max 21 tickets. There are more expensive options for €89.
viator.com offers for 49. Max 15 tickets.
Booking.com offers options from €54.90, limited to max 4 tickets
What to bring:
- Valid passport or ID (Vatican is a separate country!)
- Do not bring any sharp objects, or anything that you wouldn’t take to the airplane as you will go through airport-like security
- You can bring a 0.5 bottle of water (not sure if you can bring the large ones!)
What to wear:
- Long pants (no shorts)
- Longer skirt (no short skirts)
- T-shirt or long sleve shirts (no sleeveless shirts)