People of nearly every nationality travel to destinations abroad each year. Where they go, how long they can stay, and how their income earned abroad is taxed varies considerably, depending on the country that issued their passport.

Nomad Capitalist, an organization that helps wealthy individuals diversify assets with investments worldwide, created an index to rank countries based on the value of their passport. The group’s Nomad Passport Index 2017 is designed to show the true value of a country’s passport — not only by considering where passport holders can travel and how they will likely be perceived, but also by accounting for the places wealthy people can best avoid taxes.

24/7 Wall St. reviewed the world’s 36 most valuable passports from the Nomad Passport Index 2017. Swedish passports have the most perks, while passports from Slovenia and the United States round out the list in a tie for 35th.

Click here to see the 36 countries with the most powerful passports.

One of the most important factors in determining the power of a country’s passport is tax policy. Tax policy varies considerably from one government to another, and some countries will tax their citizens no matter where they live in the world.

For example, while Belgian and Irish passport holders earning income abroad can avoid paying taxes to their home governments relatively easily, American passport holders will always need to pay taxes to Uncle Sam, no matter where they live.

While tax policy affects a relatively small segment of the population, other index measures such as visa-free travel and dual citizenship are more widely applicable — relevant to travelers and seven-figure salary entrepreneurs alike.

Where one can travel easily is dictated almost entirely by their passport. There are 177 countries with no visa requirements for German passport holders, for example, more than there are for passport holders of any other country. Meanwhile, only 25 countries grant Afghan passport holders visa-free entry. American passport holders enjoy relatively unrestricted travel, with visa-free access to 174 countries, eighth most in the world.

Visa-free access to larger numbers of countries can certainly encourage more travel among residents. Other factors, such as geography and wealth, can also affect how frequently people travel abroad.

Small clustered European nations, for example, often report relatively high numbers of departures for international destinations in a given year. One such country, Luxembourg, where the annual number of international departures far exceeds the population, is within a six hour drive of seven other countries.

International travel is a luxury and often a costly endeavor. Most of the countries with the best passports are relatively wealthy. GDP per capita exceeds the $7,901 global average among upper middle income nations in every country on this list for which data is available. Many countries rely a great deal on international tourism. For this reason, some countries may be more readily accepting of travelers from more affluent nations.

The ease with which a citizen of a given country can obtain dual citizenship is also a critical component of a passport’s overall value. While some countries, like Canada and France, allow residents to hold dual citizenship, countries like Japan and the Netherlands expressly forbid it.

Other metrics such as the likelihood a country’s citizens will be treated with hostility abroad, were also considered in the evaluation.

To identify the 36 passports with the best travel benefits, 24/7 Wall St. reviewed Nomad Capitalist’s report Nomad Passport Index 2017. The index consists of five weighted components: visa-free travel, taxation, dual-citizenship, perception, and overall freedom. The number of countries a passport holder can visit without a visa comprised the largest weight in the index and tax-freedom accounted for the next largest. The remaining three categories were each worth an equal weight. We also considered GDP per capita and the number of departures for destinations abroad for the most recent year available from the World Bank. Annual international departures only counts residents of the country in question.

These are the countries with the best passport perks.