Ventas, Inc., an S&P 500 company, is a real estate investment trust ("REIT") with a highly diversified portfolio of seniors housing, research and innovation, and healthcare properties located throughout the United States, Canada and the United Kingdom. As of December 31, 2018, we owned approximately 1,200 properties (including properties owned through investments in unconsolidated entities and properties classified as held for sale), consisting of seniors housing communities, medical office buildings ("MOBs"), research and innovation centers, inpatient rehabilitation facilities ("IRFs") and long-term acute care facilities ("LTACs"), health systems and skilled nursing facilities ("SNFs"), and we had 19 properties under development, including five properties that are owned by unconsolidated real estate entities. Our company was originally founded in 1983 and is headquartered in Chicago, Illinois.
We primarily invest in seniors housing, research and innovation and healthcare properties through acquisitions and lease our properties to unaffiliated tenants or operate them through independent third-party managers. As of December 31, 2018, we leased a total of 442 properties (excluding properties within our office operations reportable business segment) to various healthcare operating companies under "triple-net" or "absolute-net" leases that obligate the tenants to pay all property-related expenses, including maintenance, utilities, repairs, taxes, insurance and capital expenditures.
As of December 31, 2018, pursuant to long-term management agreements, we engaged independent operators, such as Atria Senior Living, Inc. ("Atria"), Sunrise Senior Living, LLC (together with its subsidiaries, "Sunrise") and Eclipse Senior Living ("ESL"), to manage 359 seniors housing communities for us.