Although “small business owners” are often the subject of tax policy debate, a consensus does not exist regarding the specific attributes that distinguish small businesses from other firms. Previously, the Office of Tax Analysis had counted a small business owner as any individual who receives flow-through income from a sole proprietorship, partnership, S corporation, farming operation or miscellaneous rental activity. This overly broad definition was used because, for the majority of flow-through business income (partnerships and S corporations), it was not possible to trace income from the business entity to the respective owner(s). Due to newly accessible tax data, this technical constraint has been overcome. In this paper, we present a methodology that utilizes these new tax data, thereby allowing a more nuanced definition of small business owner.Our revised methodology begins with the characteristics that define a “business.” We look at the six tax forms and schedules filed by individuals or firms that could potentially represent business activity: Form 1040 Schedules C, E and F, Forms 1065, 1120 and 1120S. We develop two tests based on income and deductions reported on those forms and schedules to separate filers into business and non-business groups. We then further sub-divide our business group into small and other businesses. Drawing from various tax code provisions that provide preferential treatment to certain filers, we set the small business threshold at $10 million of income or deductions. Using these criteria, we find that 54 percent of taxpayers who file one of the six forms or schedules we consider qualify as both a business and a small business for tax year 2007. Those small businesses reported approximately 17 percent of total and net business income. We also find that slightly more than one-fifth of small businesses conform to our definition of an employer.Once we set the parameters that define a small business, we then use newly accessible tax data to separate small business income reported on the individual income tax return from other (i.e., non-small) business income. In this manner, we identify relevant characteristics of small business owners such as reported Adjusted Gross Income (AGI) and applicable marginal tax rates. For tax year 2007, our previous methodology counted 34.7 million filers reporting $662 billion of net flow-through business income as small business owners. Using our revised methodology, we count 20.0 million filers reporting $376 billion of net business income as small business owners under a broad measure of small business owner. Under our narrow definition, we count 9.4 million filers reporting $335 billion of net business income as small business owners.Because it excludes large businesses, our revised methodology also affects the distribution of net business income across AGI classes for small business owners. Using our previous methodology, eight percent of small business owners reporting 75 percent of net flow-through business income had AGI above $200,000. Using our revised methodology and our broad definition of small business owner, those shares are 11 and 64 percent respectively. Under our narrow definition, those shares are eight and 57 percent respectively.We note that our revised methodology is but one reasonable approach that could be used to identify small businesses and their owners. However, we believe it represents a significant improvement over previous methodologies that were constrained by data limitations.