Realty Income Corp (NYSE:O) is one of the largest property-owning real estate investment trusts (REITs) on the market. In addition to providing growth as the value of its properties rises, the company also provides a fantastic, growing monthly income stream to shareholders.

Here are four great reasons why Realty Income Corporation belongs in your portfolio.

Diversity, diversity, diversity
One of the most compelling reasons to buy REITs instead of individual investment properties is the diversification they provide. And, Realty income is about as diversified as they come in terms of tenants, industry exposure, and geography.

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Not only does Realty Income have a diverse group of more than 200 tenants, but the majority is made up of nationally recognized corporations, many of whom are considered "investment-grade." Realty Income's five largest tenants are Walgreens, FedEx, Dollar General, Family Dollar, and LA Fitness, and there are many more well-known companies on the list.

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Realty Income's mix of tenants is also a strategic move by the company. Management doesn't want the company's performance to depend too heavily on any one industry. So, no industry makes up more than 10% of Realty Income's revenue. In fact, the tenants represent 47 distinct industries. What this means is that if say, movie theaters started doing very poorly, only 5% of Realty Income's revenue would be in question.

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And finally, the company is very geographically diverse, with properties in every state except Hawaii. The company invests in properties in either major markets, or in strategic locations with a critical need for commercial properties.

Veteran management team
Not only does Realty Income's management team have a lot of experience in the industry, but they have been with the company for a long time.

In fact, the average member of the management team has been with the company for 17 years, and some have been around much longer. For instance, President and COO Gary Malino has been with the company since 1985.

Ambitious (but conservative) acquisition strategy
Acquiring new properties is a cornerstone of Realty Income's strategy, but the company doesn't like to go into a lot of debt to do it.

Last year, the company acquired nearly 1,000 new properties occupied by 55 tenants, and gave Realty Income exposure to five new industries, further diversifying the portfolio.

In fact, Realty Income completed $4.7 billion in acquisitions in 2013 and $3 billion of it was financed with equity, not debt. Only about 34% of the company's balance sheet is made up of debt and preferred stock, and the company has an interest coverage ratio of 3.9.

What this means is Realty Income's pre-tax earnings are almost four times the amount of the interest the company has to pay on its debt. This provides great protection in the event of weakness in the market or in a specific industry.

It's all about results
Perhaps the most compelling reason to buy Realty Income can be found in its spectacular track record of performance. Over the long run, the combination of growing income and growth in property values can produce a very nice total return.

Since going public 20 years ago, Realty Income has averaged a total return of 16.7% per year. This beat the S&P 500, which returned an average of 9.4% during the same period, and even handily beat the 10.9% total return from other equity REITs.

To put this in perspective, consider that a $10,000 investment compounded at 9.4% (the S&P's average) would grow to more than $60,000 in 20 years. At Realty Income's average return, the same investment would grow to nearly $220,000. So over time, Realty Income's consistent outperformance can really produce big gains in your portfolio.

And, Realty Income's FFO (funds from operations) per share, which my fellow contributor Patrick Morris points out is perhaps the best measure of REIT performance, grew by nearly 17% in 2013, and rose 8% year-over-year in the first quarter of 2014.

So, Realty Income is a great way to get consistent, sustainable growth, and to create a reliable monthly income stream for your portfolio. The diversification of the portfolio and experience of the management have produced results that speak for themselves.