Muslims make up the world's second-largest religious group, after Christians, with an estimated 1.8 billion Muslims worldwide as of 2015. And the religion of Islam is growing faster than any in the world, according to the Pew Research Center.
Which countries have the most Muslims per capita? According to Pew, the CIA World Factbook and other sources, here are the most heavily Muslim countries on Earth.
Jordan's population is 93.8 percent Muslim.
Here, a Jordanian girl holds up the Muslim holy book, the Quran, during a rally.
Mali's population is 95 percent Muslim.
Here, Muslim faithful gather for the evening prayer in the northern Malian city of Gao.
Gambia's population is 95.3 percent Muslim.
Kosovo's population is 95.6 percent Muslim.
Here, young Kosovo Muslims attend Eid al-Fitr prayers, marking the end of the holy month of Ramadan.
Senegal's population is 95.9 percent Muslim.
Here, a member of the Mouride Brotherhood, a large Sufi sect common in Senegal, reads a religious book outside the Great Mosque in Touba.
Pakistan's population is 96.4 percent Muslim.
The population of Uzbekistan is 96.5 percent Muslim.
Here, Uzbek Muslims perform their midday prayers in a local mosque.
Libya's population is 96.6 percent Muslim.
Here, Libyan Muslims pray on the first day of Eid al-Adha, or the Festival of Sacrifice, which marks the end of the Hajj pilgrimage to Mecca.
The population of Tajikistan is 96.7 percent Muslim.
Azerbaijan's population is 96.9 percent Muslim.
14 (TIE). Sudan
Sudan's population is 97 percent Muslim.
14 (TIE). Djibouti
The population of Djibouti, in the Horn of Africa, is 97 percent Muslim.
In this photo, worshippers bow during an evening service in Djibouti Town.
13. Saudi Arabia
Saudi Arabia's population is 97.1 percent Muslim. The Sunni branch of Islam is the official state religion.
Like several other countries at the top of this list, Saudi Arabia's law requires that all citizens adhere to Islam. However, a portion of the population is made up of foreign workers who aren't necessarily Muslim.
Still, "freedom of religion is neither recognized nor protected under the law" even for these foreign workers, and no religions other than Islam are allowed to be practiced openly, according to a U.S. State Department report.
Algeria's population is 98.2 percent Muslim.
The population of Niger is 98.3 percent Muslim.
Turkey's population is 98.6 percent Muslim.
Here, Turkish people prepare to break their Ramadan fast in front of the Blue Mosque in Istanbul.
The population of Somalia is 98.9 percent Muslim.
Here, Somali Muslims read the Quran on the second day of the holy month of Ramadan.
6 (TIE). Yemen
Yemen's population is also 99 percent Muslim.
A Yemeni boy reads from the Quran during the holy Muslim fasting month of Ramadan.
6 (TIE). Morocco
Morocco's population is also 99 percent Muslim.
Moroccan worshipers in this photo pray during Laylat al-Qadr, which falls on the 27th day of the fasting month of Ramadan.
6 (TIE). Iraq
Iraq's population is 99 percent Muslim. The country has large groups of both Sunni and Shia Muslims.
In this photo, Iraqis wish each other happy Eid al-Adha following early morning prayers in the Umm al-Qura mosque in Baghdad.
Iran's population is 99.7 percent Muslim, primarily Shia.
Here, Iranians perform night prayers outside Emamzadeh Saleh mosque during the holy month of Ramadan.
3 (TIE). Tunisia
Tunisia's population is 99.8 percent Muslim.
Here, Tunisian Muslim women at morning prayers at the Zitouna mosque in Tunis during Eid al-Fitr, marking the end of Ramadan.
3 (TIE). Afghanistan
The population of Afghanistan is 99.8 percent Muslim.
Here, worshippers attend special prayers in celebration of the first day of the Eid al-Fitr festival, marking the end of the holy month of Ramadan.
1 (TIE). Mauritania
Since 1960, Mauritania has been an Islamic Republic, and 100 percent of the population is Muslim. Most of the population are Sunni.
1 (TIE). Maldives
The population of the Maldives is 100 percent Muslim. Islam is the state religion of the country, and citizens are legally required to follow it. As of 2008, the law stated that a non-Muslim may not be a citizen.