The United States of America is often called the "land of opportunity," a place where hard work and sacrifice lead to economic success. Across generations, countless families have been able to live out that promise. However, more than one out of four American working families now earn wages so low that they have difficulty surviving financially. These are families with responsible, hard-working breadwinners who want to get ahead but hold down low-paying jobs with inadequate benefits and little hope for advancement. Many lack the skills and education they need to move into jobs that pay better, even while the economy demands more highly trained employees. And while the nation's economy relies on the service jobs these low-paid workers fill--such as cashiers, janitors, security guards and home health aides--society has not taken adequate steps to ensure that these workers can make ends meet and build a future for their families, no matter how determined they are to be self-sufficient. The Working Poor Families Project, with the support of the Annie E. Casey, Ford and Rockefeller foundations, has spent three years working in 15 states to examine both the conditions of low-income working families with children and public policies that can help improve their lives. This report builds on that work and takes a broader look at low-income working families across the nation and key facets of an economic and public system that affect their ability to achieve economic security. Appended is: Defining Economic Self-Sufficiency Realistically. (Contains 4 tables, 2 charts, and 67 endnotes.) [This report was written with assistance from Sara Rab and Steve Ressler. It is a product of the Working Poor Families Project, a national initiative supported by the Annie E. Casey, Ford and Rockefeller foundations.]