Sixth Street Specialty Lending is a specialty finance company focused on lending to middle-market companies. Since we began our investment activities in July 2011 through September 30, 2021, we have originated more than $17.5 billion aggregate principal amount of investments and retained approximately $7.7 billion aggregate principal amount of these investments on its balance sheet prior to any subsequent exits and repayments. We seek to generate current income primarily in U.S.-domiciled middle-market companies through direct originations of senior secured loans and, to a lesser extent, originations of mezzanine and unsecured loans and investments in corporate bonds and equity securities. The companies in which we invest use capital to support organic growth, acquisitions, market or product expansion and recapitalizations. 98.9% of debt investments bore interest at floating-rates (when including investment specific hedges), which we believe helps act as a portfolio-wide hedge against inflation. As of September 30, 2021, Sixth Street Specialty Lending portfolio had a fair value of approximately $2,406.5 million invested across 67 portfolio companies.
We have elected to be regulated as a business development company, or a BDC, under the Investment Company Act of 1940 and the rules and regulations promulgated thereunder. We are externally managed by Sixth Street Specialty Lending Advisers, LLC, an affiliate of Sixth Street and a Securities and Exchange Commission (“SEC”) registered investment adviser. We leverage the deep investment, sector, and operating resources of Sixth Street, a global investment firm with approximately $48 billion of assets under management.