Small business size standards are of congressional interest because the standards determine eligibility for receiving Small Business Administration (SBA) assistance as well as federal contracting and tax preferences. Although there is bipartisan agreement that the nation's small businesses play an important role in the American economy, there are differences of opinion concerning how to define them. The Small Business Act of 1953 (P.L. 83-163, as amended) authorized the SBA to establish size standards for determining eligibility for federal small business assistance. The SBA currently uses two types of size standards to determine SBA program eligibility: industry-specific size standards and alternative size standards based on the applicant's maximum tangible net worth and average net income after federal taxes. The SBA's industry-specific size standards determine program eligibility for firms in 1,047 industrial classifications in 18 sub-industry activities described in the North American Industry Classification System (NAICS). The size standards are based on one of four measures: (1) number of employees, (2) average annual receipts in the previous three years, (3) average asset size as reported in the firm's four quarterly financial statements for the preceding year, or (4) a combination of number of employees and barrel per day refining capacity. Overall, the SBA currently classifies about 97% of all employer firms as small. These firms represent about 30% of industry receipts. The SBA has always based its size standards on economic analysis of each industry's overall competitiveness and the competitiveness of firms within each industry. However, in the absence of precise statutory guidance and consensus on how to define small, the SBA's size standards have often been challenged, typically by industry representatives seeking to increase the number of firms eligible for assistance and by Members concerned that the size standards may not adequately target assistance to firms that they consider to be truly small. During the 111th Congress, P.L. 111-240, the Small Business Jobs Act of 2010, authorized the SBA to establish an alternative size standard using maximum tangible net worth and average net income after federal taxes for both the 7(a) and 504/CDC loan guaranty programs. It also established, until the SBA acted, an interim alternative size standard for the 7(a) and 504/CDC programs of not more than $15 million in tangible net worth and not more than $5 million in average net income after federal taxes (excluding any carry-over losses) for the two full fiscal years before the date of the application. It required the SBA to conduct a detailed review of not less than one-third of the SBA's industry size standards every 18 months. This report provides a historical examination of the SBA's size standards and assesses competing views concerning how to define a small business. It also discusses H.R. 527, the Small Business Regulatory Flexibility Improvements Act of 2015, which would authorize the SBA's Office of Chief Counsel for Advocacy to approve or disapprove a size standard requested by a federal agency for purposes other than the Small Business Act or the Small Business Investment Act of 1958. The SBA's Administrator currently has that authority. This report also discusses P.L. 112- 239, the National Defense Authorization Act for Fiscal Year 2013, which requires the SBA to make available a justification when establishing or approving a size standard that the size standard is appropriate for each individual industry classification. It addresses the SBA's recent practice of combining size standards within industrial groups as a means to reduce the complexity of its size standards and to provide greater consistency for industrial classifications that have similar economic characteristics.