The first thing to know is that the ticket for the Colosseum also gets you into the Forum and the Palatine Hill, and vice versa.
Secondly, the Palatine and the Forum are in the same complex, and you access them through the same entrance (more on that later), while the Colosseum has to be entered via a separate entrance: a huge door in the base of the massive stadium.
So, the cheapest way to buy a Colosseum ticket is to go direct to the official Colosseum website and buy the ticket there. Despite the odd web address, that is in fact the official website.
An alternative would be to support this site and buy your Colosseum ticket here instead (this costs you €1 more than the official site, but every ticket sold earns us a small commission).
Whichever way you buy the ticket you’ll be emailed a PDF voucher, and then you will need to print it out. This voucher needs to be converted into a ticket at the first site you visit.
For the Colosseum this can be done at any of the ticket-holders-only booths just inside the Colosseum door. However during peak months of the year there are temporary booths on the other side of the street from the Metro station that will do this too. If you are entering the Forum/Palatine complex first, you do this at the main Forum ticket booth, indicated on the map. Note that the lines here are usually much shorter.
Note that the Colosseum operates a strict capacity control of 3,000 people at any one time, which during peak times can affect your waiting time even if you have a ticket.
Insider tip: if you haven’t been able to buy a Colosseum ticket in advance, we recommend making your way to the ticket office at the Forum entrance (marked on the map above: down the lane opposite the end of Via Cavour) where the lines are much less brutal, and you can buy a combined ticket there without so much waiting in line.