A fifth group, the Ijaw, has been growing in population and influence and currently makes up another 10 percent.
Muslim Hausa and Fulani are the predominant ethnic groups in Nigeria's northern region. Though the groups originated in different parts of West Africa, religion, intermarriage and adoption of the Hausa language by the Fulani have unified the groups over time. In contemporary Nigerian society, they are often referred to collectively as Hausa-Fulani.
The largest of the major ethnic groups, Hausa and Fulani have been politically dominant since Nigeria's independence from Britain in 1960.
Islam is a key component of their ethnic identity and continues to inform their role in modern Nigerian society and politics. Their culture is deeply patriarchal and patrilineal.
In recent years, Hausa and Fulani were instrumental in adopting and upholding Sharia, a system of Islamic law, in 11 of the country's northern states.
The Igbo, the main ethnic group in southeastern Nigeria, has represented some of the staunchest opponents of Sharia law. In many northern Hausa-Fulani-dominated states, minority populations of Igbo claim to have been unfairly targeted by laws that do not pertain to their faith.
Unlike neighboring Hausa and Yoruba cultures, Igbo society was traditionally decentralized and non-hierarchical. This made its members easier converts for European missionaries and today most Igbo are Christian.
Under British colonial rule, many Igbo served in government and military roles and were later key players in Nigerian independence. But over the last few decades the group has become less politically dominant.
Discovery of large oil reserves near Igboland in the early 1960s and proposed redistricting led many in the group to fear that they would be cut out of revenues from the country's natural resources.
In 1967, an Igbo secessionist movement in Biafra state led to a 30-month war with the Nigerian government, in which hundreds of thousands of Igbo starved to death.
After the war, Igbo were reintegrated into Nigerian society, but in a more marginalized role. Despite lingering ethnic tension, they now play an important part in southeastern Nigeria's oil trade. In recent elections, however, they have struggled to coalesce around a single candidate for the presidency.
The Yoruba are one of Nigeria's most urban ethnic groups. Historically, their culture centered on densely populated city-states each controlled by an oba, or king. Yoruba now form the majority in Lagos, the second most populous city in Africa.
In modern day Nigeria, Yoruba speakers do not always identify with their larger ethnic group, but rather the many smaller Yoruba-speaking communities.
This pluralism extends to Yoruba views of religion. As Islam and Christianity spread to Yorubaland over the past few centuries, the group embraced both faiths alongside its many traditional and animist beliefs. This blend and acceptance of religion survives in modern times and has mitigated some religious conflict in places where Yoruba form the majority.
Like the Igbo, Yoruba held important roles in the British colonial government, participating significantly in both political and economic life. Since independence, the group has been overshadowed by the more numerous and dominant Hausa-Fulani.
However, in 1999 a Christian Yoruba named Olusegun Obasanjo became Nigeria's president and first elected head of state. He was reelected for a second term in 2003.
In recent years, the Ijaw have agitated for more political franchise in Nigeria. Although they are the fifth largest ethnic group in the country, their traditional lands in the Niger River Delta are some of the country's most oil-rich.
Oil exploration has had devastating consequences on Ijaw territory and subjected the group to numerous ecological hazards. Mismanagement of oil revenues has kept much of the wealth from returning to Ijaw communities.
In January 2006, the Ijaw militia Movement for the Emancipation of the Niger Delta kidnapped four Royal Dutch Shell employees in the Niger Delta region, demanding the release of an Ijaw militia leader who was arrested by Nigerian authorities. His continued detention has caused members of MEND to swear continued attacks and disruptions to the oil industry.
Despite these ongoing tensions, 2007 could see an Ijaw take a major political office for the first time. Goodluck Jonathan, an Ijaw, is running as a vice presidential candidate for Alhaji Umaru Musa Yar'adua, one of the front-runners.
The Nigerian government has worked out tentative power-sharing arrangements to help ensure that its many ethnic groups have some say in how the country's natural resource wealth is spent. But major questions about ethnicity and how to balance the many competing interests still dog the society.
Cities remain largely segregated along ethno-religious lines, and confrontation between ethnic groups is common. Often, ethnic clashes in one part of the country can set off a chain of reprisal riots and attacks in other parts of the country.
All major ethnic groups have formed militias to protect their own interests and perpetrate violence on other groups. While illegal, these vigilante groups continue to act with impunity for lack of stringent law enforcement.