What permits do I need to trek to Everest Base Camp?
Everything you need to enter Nepal and trek to Everest Base Camp as a non-citizen.
You're gonna need a passport obviously. Maybe less obviously, make sure it doesn't expire anytime soon, especially not within the next six months.
Nepal entrance visa
All non-citizens of Nepal need to obtain an entrance visa upon arrival at Kathmandu, click This Blog Post for more information about the entrance visa process.
You can get a 15 day visa for $25 USD (they accept American dollars, I think cash only...so bring cash). You can also get a 30 day visa for around $40-50 dollars.
You also need a physical passport-style photo to hand to the guy making your Nepal entrance visa. They don't really care how precise it is, as long as it looks somewhat like how a passport photo is supposed to look.
Sagarmatha National Park entrance permit
You buy this permit as you enter the Everest Base Camp trail near Lukla. It costs 30 USD equivalent, they will accept the payment in Nepalese Rupees. There will be Nepal soldiers in a couple places along the path looking to see your permit. Both times I ran into these guys, I had no idea what they were looking for, they kept calling it a 'Tames, or Tims card or something and I couldn't understand. Eventually they just let me go on my way anyway, we are bros for life.
Despite the fact that things are pretty cheap everywhere in Nepal, I kept running out of Rupees. So make sure you have enough at the beginning of your trek. Also make sure to exchange them back somehow before you leave Nepal. I didn't have time to exchange mine back and now I'm stuck with $200 dollars worth of Rupees because no American airport exchanges them...