Rome has three metro lines for twenty centuries of city. It sounds like a problem and it is actually the key to planning the trip: once you know what the metro drops you at and what it does not, you stop losing entire mornings.
The network has three lines — A, B and C, plus the B1 branch — and they meet at exactly two points: Termini (A with B) and San Giovanni (A with C). That is it. For a city this size it is a tiny network.
The reason is the one you are imagining: every time anyone digs in Rome something turns up, work stops and the line takes decades. Line C has been under construction since 2007 and still has not reached the historic centre.
The practical consequence is what matters: much of the historic centre has no metro. The Pantheon, Piazza Navona, Campo de’ Fiori, Trastevere and the Trevi Fountain have no station of their own. The Colosseum and the Vatican, on the other hand, it drops you right at the door.
So the real split of a day in Rome is: metro for the long hops, and walking — or tram 8 — for the centre. Once you accept that, the city gets enormously easier.

Ordered by what we think is most worth it, not by what comes top of the lists. Each one with its stop.
Everything above you can do on your own with this page open. This other thing we build for you: the same idea as always — you do not know what you got until the day before — applied to Rome.
You choose where you fly from and where you are going, and we tell you what the adventure costs and roughly what getting there would cost. Flights and accommodation you book yourself: we are not a travel agency and we do not want to be.
Rome has one virtue you will be grateful for: a flat fare. There are no zones inside the city, so the ticket costs the same whether you go two stops or cross the whole network.
That €8.50 daily cap is why a tourist pass almost never pays off if you are tapping a card: paying per journey you never spend more than that in a day, and on many days you will spend a good deal less.
If you want it line by line — which stations each one has, what is around every stop and how to work out a specific journey — we have all of it at our guide to the Rome metro. It is one of the sites we publish ourselves, and it is free.
Fiumicino has two trains and the difference between them is six euros and which station you want to arrive at.
Three full days cover the Colosseum, the Vatican and the centre without running. The fourth is the one that gets interesting: Testaccio, the Appia or Ostia Antica.
A single BIT ticket costs €1.50 and is valid for 100 minutes. Paying contactless costs the same per journey and caps at €8.50 in 24 hours, which is usually the cheapest way.
Only partly. The Colosseum and the Vatican, yes. The Pantheon, Piazza Navona, Trevi and Trastevere have no station: those you reach on foot or with tram 8.
Only if you are going to use the museum entries it includes. For transport alone it does not pay off, because the contactless daily cap already limits what you spend.
April to early June, and late September to October. July and August are very hot and the Forum has no shade. August also empties the city of Romans, which changes the neighbourhoods.
This is what we do when we travel. The other thing we do is build adventures where you do not know which activity you got until the day before.