Irena Sharoshkina Boostani came to the United States in 1998 with $450 and few plans.

After growing up in a small town in Russia, she met a Peace Corps volunteer from California who told her she should consider moving to America. As soon as she could, Boostani headed for Los Angeles.

Boostani, now a 35-year-old mother of two, has dual citizenship in Russia and the United States after marrying an American citizen.

"The duality is really an opportunity to bring the two homes together," said Boostani, who lives in San Diego. "My existing home is definitely America -- there is no question in my mind. My mom and dad are still in Russia, and having that dual citizenship is peace of mind I can go back and forth. It feels free, it really does."

It is nearly impossible to estimate how many U.S. citizens have dual -- or even triple -- citizenships, says Michael A. Olivas, an immigration professor at the University of Houston Law Center.

It's like estimating "how many people in the United States have blue eyes," he says, because there is no single registry of these individuals, and the numbers change so rapidly.

The number is likely well over 1 million, he says, and is probably several times that.

For those millions, the road to holding two passports is not always simple.

The United States does not formally or officially recognize dual citizenship, says Daniel Cosgrove, a spokesman for the U.S. Citizenship and Immigration Services.

During the naturalization ceremony, individuals officially become U.S. citizens when they take the Oath of Allegiance to the United States. The text of the naturalization oath became regulation in 1929 and states, "I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state or sovereignty, of whom or which I have heretofore been a subject or citizen."

In practice, the naturalization oath means if naturalized citizens were ever faced with a choice of allegiance to the United States or a different nation, they have an obligation to choose the United States, says Michael Wildes, a partner in the New York-based immigration firm Wildes & Weinberg, which has represented high-profile clients including John Lennon and Yoko Ono.

The understanding, Wildes says, is a U.S. citizen will "work in concert with the U.S. Constitution but never against it."

Pledging allegiance

There are large contingents of U.S. citizens who are also citizens of Canada, Mexico and the United Kingdom, Olivas says, though dual citizens represent a host of other countries.

Charles Foster, a former senior policy adviser to George W. Bush and a managing partner at FosterQuan, one of the largest immigration law firms in the United States, says he tells clients dual citizenship can be viewed as "two for the price of one."

"You don't have to make an agonizing decision to give up citizenship. You can have both," he says.

According to the U.S. State Department website, federal law "does not mention dual nationality or require a person to choose one citizenship or another."

It is possible to become a dual citizen in the United States in several ways, says Catherine Wilson, an associate professor of political science at Villanova University who specializes in immigration politics.

These include being born in the United States to at least one immigrant parent whose home country grants the child automatic citizenship, or being born in certain countries to at least one parent who is a U.S. citizen.

If not a citizen by birth, people can go through the naturalization process in the United States and remain dual citizens if they are not required to renounce citizenship in their countries of origin.

Issues of dual citizenship become complicated because they are unique to each country and depend on the relationship each has with the United States.

Although many countries allow dual citizenship with the United States, several require renunciation of citizenship in the country of origin if an individual becomes naturalized as a U.S. citizen -- a process known as forced expatriation. Those countries, according to the Migration Policy Institute, include Venezuela, the Dominican Republic, Kenya, Botswana, Afghanistan, Indonesia, Japan, the Philippines, Thailand and India, among others.

In general, it is harder to gain dual citizenship with a country the United States "does not have a favorable relationship with, or is at war with," Wilson says.

Since the founding of the United States, leaders have been concerned about Americans' political loyalty, Wilson says.

There was "the concern that even people who would be our leaders would have these foreign allegiances that would lead them astray and have them sympathize on behalf of foreign powers," she says.