The High School Longitudinal Study of 2009 (HSLS:09) is the fifth in a series of National Center for Education Statistics (NCES) secondary longitudinal studies. The core research questions for HSLS:09 explore secondary to postsecondary transition plans and the evolution of those plans; the paths into and out of science, technology, engineering, and mathematics; and the educational and social experiences that affect these shifts. The HSLS:09 base year took place in the 2009-10 school year, with a randomly selected sample of fall-term 9th-graders in more than 900 public and private high schools with both a 9th and an 11th grade. Students took a mathematics assessment and survey online. Students' parents, principals, and mathematics and science teachers and the school's lead counselor completed surveys on the phone or on the Web. The first follow-up of HSLS:09 will take place in the spring of 2012 when most sample members will be in the spring of their 11th grade. Dropouts and transfer students will be followed, as well as those who remain in the base-year school. A postsecondary update will take place in the summer of 2013, to learn about the cohort's postsecondary plans and decisions. High school transcripts will be collected in the fall of 2013, and a second follow-up will take place a few years beyond high school graduation. Further information on study design and purposes can be found in chapter 1 of this document. This manual has been produced to familiarize data users with the design, and the procedures followed for data collection and processing, of the base year of the High School Longitudinal Study of 2009 (HSLS:09). It also provides the necessary documentation for use of the public-use data files, and information that will be helpful to analysts in accessing and understanding the restricted-use files. Chapter 1 serves as an introduction to HSLS:09. It includes an overview and history of the National Center for Education Statistics (NCES) program of longitudinal high school cohort studies, summarizes the HSLS:09 objectives, and supplies an overview of the base-year and longitudinal study design. Chapter 2 describes the base-year data collection instruments, including both the development and content of the student, parent, science and mathematics teacher, counselor, and school administrator questionnaires. Chapter 2 also provides information on the development of the direct algebra assessment and the scoring procedures and psychometric characteristics. The sample design used in the base year is documented in chapter 3. Data collection methods and results--including schedules, training, procedures, and response rates--are presented in chapter 4. Chapter 5 describes data preparation and processing, including the receipt control system, coding operations, machine editing, and data file preparation. Additionally, chapter 5 provides information on the data preparation, scaling, and psychometric characteristics of some of the scales used in the student, teacher, school administrator, and school counselor surveys. Chapter 6 describes weighting, variance estimation, and unit nonresponse bias estimation, while chapter 7 examines item-level statistical issues such as item nonresponse bias, imputation, and disclosure risk analysis. Chapter 8 describes the contents of the data files, including the data structure and linkages to other databases. The following are appended: (1) Base-Year Questionnaires; (2) HSLS:09 Mathematics Assessment Items: 2009; (3) Glossary of Terms; (4) Details of School and Student Sampling; (5) Parental Passive and Active Consent Forms; (6) Documentation for Composite Variables; (7) Standard Errors and Design Effects; (8) Unit and Item Nonresponse Bias Analyses; (9) Imputation Details; (10) Socioeconomic Status; (11) Base-Year Codebooks; (12) Comparison of Public and Restricted-Use Files; and (13) Variable List for the HSLS:09 Electronic Codebook (ECB).