On Tuesday, after 20 months of arduous negotiations, Iran and the P5+1, comprised of the United States, Russia, China, France, Great Britain and Germany, reached an agreement to limit Iran's ability to produce a nuclear weapon in exchange for easing sanctions. 

The deal, which is intended to prevent a nuclear arms race in the Middle East, is extremely controversial. Iran's regime is a longstanding enemy of the United States. The country is a sponsor of armed groups in Iraq, Syria, Lebanon, Yemen and Gaza and an avowed opponent of Israel; Israeli Prime Minister Benjamin Netanyahu considers Iran to be a threat to the Jewish state's existence. Republicans generally oppose the deal, and Congress will have 60 days (interrupted by a month-long recess) to review it. President Obama has vowed to veto any legislation that would prevent the deal from going into effect. 

And yet, as politicians and diplomats battle tooth and nail in several world capitals, the business world is all smiles. Iran's population is large, at around 80 million; young, with a median age of 28; well-educated; and hungry for consumer goods that have long been off limits. Its economy, though ravaged by imposed isolation, is still fairly diversified. Its decaying infrastructure is ripe for investment. Its energy reserves are vast. For Western companies, some kicked out as recently as 2012, the deal is a mouthwatering opportunity.

Assuming Iran does open up to foreign investors, here are three major changes to look out for.

1. Oil Prices

Iran has the world's fourth-largest oil reserves. The United States has imposed sanctions on the Iranian energy sector since 1995, and the EU banned petroleum imports from the country in 2012, causing crude oil production in the country to drop from 3.6 million barrels per day in late 2011 to an estimated 2.9 million barrels per day in June 2015.

Iran's oil-producing infrastructure needs to be upgraded, and the timing of sanctions relief depends on the results of International Atomic Energy Agency inspections. Still, the U.S. Energy Information Administration has estimated that Iranian supply could drive the price of crude oil down by $5 to $15 per barrel from where it might otherwise be in 2016.

2. Head Start For European Companies

Of the companies hoping to exploit Iran's oil reserves, Total SA (TOT), Eni SpA (E), StatOil AS A (STO), Royal Dutch Shell PLC (RDS-A) and BP PLC (BP) are in a better position than their U.S. counterparts. Shell, Eni and Total executives were already meeting with officials in Tehran in May and June in anticipation of sanctions relief. Meanwhile ConocoPhillips Co (COP) stated that it is not involved in discussions with Iran, while Exxon Mobil Corp (XOM) declined to comment.

European firms also have a head start in other Iranian sectors where foreign companies are itching to invest, such as tourism and auto manufacturing. The primary reason is that, while most if not all EU sanctions will be lifted by Tuesday's agreement, many U.S. sanctions predate revelations about Iran's nuclear program and will remain in place.

The disadvantage for U.S. companies can be seen in the tourism sector. Hilton Worldwide Holdings, Inc, (HLT), Hyatt Hotels Corp. (H) and Sheraton, a Starwood Hotels & Resorts Worldwide Inc. (HOT) brand, have not operated in Iran since the 1979 Iranian Revolution. Meanwhile, the French hotel operator Accor SA began doing business in Tehran earlier this year.

Auto manufacturing tells a similar story: Paris-based Peugeot SA is in partnership talks with Iran Khodro and hopes to sell 400,000 cars in the country by 2025, if not 2020. General Motors Co. (GM), which as part-owner of Peugeot pressured the car maker out of Iran in 2012, has declined to comment on whether it will enter the market.

All is not lost for U.S. companies, however. Boeing Co. (BA) began doing business in Iran last year and may be well positioned to help Iran replace up to 400 planes. General Electric Co. (GE) is already selling medical equipment in the country under a humanitarian exemption to sanctions. Coca-Cola Co. (KO) sells concentrate to a local company, which in turn bottles it, but the Atlanta-based company has no tangible assets in the country.

The United States's globally recognizable consumer brands will probably make it into Iran eventually, helped along by the forbidden fruit allure the anti-American regime has fostered. Apple Inc. (AAPL) is quietly engaged in talks with local distributors, and iPhones, acquired surreptitiously or in Dubai, are already a common enough sight in Tehran. For now, consumer giants such as Starbucks Corp. (SBUX) and McDonald's Corp. (MCD) are keeping mum, but the availability of an application for Iranian McDonald's franchises shows which way the wind is blowing. 

3. Iranian Equities

For those who would like truly direct exposure to post-sanctions Iran, there is the Tehran Stock Exchange, which combines the unrealized potential of an emerging market with a dose of developed-market professionalism. A number of Western funds are eyeing Iranian equities, and in the event that sanctions are lifted, a significant amount of foreign capital will probably flow to Tehran; currently foreign capital represents 0.1% of the exchange's $150 billion market capitalization. When this might be possible, particularly for American investors, is anyone's guess, but in any event there's an ETF waiting. Tantalizingly, there may even be a value opportunity, as a big sell-off in 2014 caused the exchange to post its first yearly loss since 2008.

The Bottom Line

Iran's economy was plunged into depression not by its own failures, but by politics. In that sense, it represents an exciting investment opportunity. On the other hand, the same hard-line theocratic regime that drew the world's ire in 2002—and ire from the United States from day one—remains in power. It could sap investors' gains through corruption and red tape, or by backpedaling on the deal's terms and causing a snapback of sanctions. Or it could go all out and spark a nuclear war. Investors might not be comfortable contributing to Iran's armed proxies' coffers, particularly when their taxes are already arming the other side (or, perhaps, all sides even knows). Even if the deal goes ahead, EU investors and companies will have better access than their American peers. But whatever happens, big, brainy, oil-rich Iran is worth keeping an eye on.

Want to learn how to invest?

Get a free 10 week email series that will teach you how to start investing.

Delivered twice a week, straight to your inbox.