The National Assessment of Educational Progress (NAEP) is the largest continuing and nationally representative assessment of what students in public and private schools know and can do in various subjects. First administered in 1969, NAEP collects and reports information on student trends and performance and compares the achievement of students in public schools with that of students in private schools. Private schools represent about 25 percent of the schools in the nation, educate approximately 9 percent of the nation's students, and are a critical component of the NAEP program. Private school data are essential to providing a complete and accurate picture of education in the United States and to informing future policy decisions. The results of NAEP are released to the public as The Nation's Report Card. This brief document highlights the essential partnership between private school teachers and NAEP, the 2019-2020 NAEP program, how NAEP reports results, and results of the NAEP long-term trend mathematics assessments between 1978 and 2012.