The National Longitudinal Transition Study 2012 (NLTS 2012) is the third in the series of NLTS studies sponsored by the U.S. Department of Education to examine youth with disabilities receiving services under the Individuals with Disabilities Education Act (IDEA), a long-standing federal law last updated in 2004. Under IDEA, youth with disabilities can be eligible to receive special education and related services through an individualized education program (IEP). The NLTS studies have used survey and administrative data to describe the backgrounds of youth with an IEP and their functional abilities, activities in school and with friends, academic supports received from schools and parents, and preparation for life after high school. The NLTS 2012 was designed to address three sets of questions about youth with an IEP and their experiences, comparing different groups of youth. This design documentation describes how the NLTS 2012 data were constructed and prepared for analyses. Specifically, it provides information on the NLTS 2012 sample design, the data collection instruments and procedures, and the preparation of the data and analytic weights. It also includes findings from analyses of the potential for nonresponse bias. This report does not cover the ways the NLTS 2012 data were reweighted and combined with data from the NLTS and the NLTS 2 to examine trends for youth with an IEP. The Volume 3 report provides this information, including more detail on steps to make analytic variables and samples comparable, response rates across the studies, and weighting adjustments. Appendices include: (1) The Parent Survey Instrument; (2) The Youth Survey Instrument; (3) Skip Logic Errors in the Surveys; (4) Supplemental Tables of Standard Errors and Design Effects; (5) Supplemental Tables for the Unit-Level Nonresponse Bias Analysis; and (6) Supplemental Tables for the Item-Level Nonresponse Bias Analysis.