It's not altitude meds, drinking water or sturdy, flat-soled shoes.  All of which are recommended.  (Although altitude meds are optional. Machu Picchu tops out around 8000 ft. / 2438 m. Which is not much compared to Cusco at 11000 ft / 3350 m.) By the time you go through Cusco and the Sacred Valley, which will squander about 5 hours of travel time to get to Machu Picchu, you should feel about as good as you can expect to feel if you're coming from sea level.  If you get addled by the altitude, and forget everything else, try to burn this one important thing into your oxygen-deprived brain.

What's the most important thing you need to enter Machu Picchu?

If you are a foreigner, you need a passport to enter Machu Picchu. South American nationals are allowed to use their national ID. And everyone must buy a ticket in advance. No entry tickets are sold at the main entrance to Machu Picchu anymore. Tickets can be bought online or at the office in Cusco or Aguas Calientes. And you need to use your unique document number to get the ticket. This is not like Kayak.com or Expedia where you merely write in the name, DOB and pay with your credit card.  To buy a ticket you need to have your document. And when you enter Machu Picchu, you must show your original passport and your valid ticket. Photo copies are not accepted. However, on rare occasions when we have had groups show up at the main entry and forget their tickets, the guide has gone to the customer service office with their passports and they have printed copies of the entry tickets. Luckily everything is booked electronically and your name and passport number are stored in the government database. What if your passport gets lost, stolen or you forget it? It's essential that you file a police report which you will need to show to the agents on duty that day.  At one time or another, all of these situations have happened to most of our guides, who advocate for you with the agents on duty to work out solutions.  But there are no guarantees.  If you are without the service of a guide (who is like your Mom on steroids!) you're on your own to communicate your needs. Or beg for mercy. 

Are other documents accepted?

For foreigners, a valid passport for at least 6 months beyond the end of your trip is the only document accepted.  Passport cards are not accepted.  Drivers' licenses and birth certificates are not accepted.  ISIC (student card) is not accepted. (but can be used for discounts in some places including Machu Picchu)  South American nationals can use their national ID.  

What if I don't have a passport?

Simply put, you will not be able to reserve flights, trains, permits or official entry tickets to anything.  It may be possible to reserve a hotel if you are in contact with a real person who has the authority to hold the reservation until your passport is issued.  We do this all the time with groups where 1 or more people may be waiting on their document.  We've known people who booked trips with us then wait to apply for their passports thinking they had several months before their trip. This hampers our efforts to buy flights and entry tickets in advance and it can lead to increased costs. 

What if my Passport is Due to Expire?

The good news is that if you have an expired passport or a passport that is due to expire, it is possible to book everything using the expired document, as long as you understand that you absolutely must travel with both the old expired document and the new passport. The bad news is that if you forget your old passport, all the tickets booked with that document are useless.  When you apply for a new passport, you will mail your old passport to the government agency in charge of renewals. Once the new passport is mailed, they will also return the old document.  Note that it is customary to send the old and new documents separately.  The date of expiration of the old passport doesn't seem to matter.  We've used documents as old as 2007.  

Can I use a photo copy or electronic copy of my passport?

No. With this exception.  In an emergency, it may be possible to use an electronic copy to enter Machu Picchu or board the train. But this is determined by factors that are out of our control. This is why we ask people to email copies of their documents.  In a couple of cases, a hiker has left his/her passport with their excess luggage stored in the hotel while they hike.  We have emailed the digital copy of their passport to their guide to get them on the train and into Machu Picchu.  Here's the sad part.  We cannot use it to get them on the Inca Trail. The rigid rules require that hikers must show their original document at the point of entry.  Not even God can get you on the Inca Trail without your original passport.

What if I make a mistake with my online reservation?

Please don't make a mistake.  Corrections must usually be handled in person, which doesn't work if you're in North America or Europe.  We can arrange to send a guide to try to fix it. We've had cases where the agency in Cusco not only required the guide to present a copy of the valid passport, but they also required a copy of the credit card used to pay for the transaction.  This means you have to send us a photo of your credit card which we in turn forward to our guides in Cusco who present it as evidence to validate the correction. Some agencies require us to provide an officially signed letter explaining the correction.  

Tips to avoiding problems with online reservations

If you are booking through an agent or tour operator, they will deal with the hassles for advance purchase of tickets and permits.  However there is a limit to what they can do if you show up without the proper documents.  Keep your passport up to date to avoid any confusion about which documents you need to travel with. And if you don't understand how to use online booking services, try to hook up with a tech-savvy friend (ask any 7 year old kid!) 

In case you're wondering, there's still plenty of old-fashioned, cash-only business transactions in Peru.  All Inca Trail permits must be paid for in person with dirty, cold cash.