For more than 40 years, policymakers have committed to supporting the education of students with disabilities, who have grown as a share of all students in the United States (Snyder, de Brey, & Dillow, 2016). Beginning with landmark legislation in 1975, the U.S. Congress mandated that students with disabilities have access to a free and appropriate public education and provided funds to school districts nationwide to help serve them. Since then, the legislation has been updated six times, most recently in the 2004 Individuals with Disabilities Education Act (IDEA), which emphasized helping youth prepare for postsecondary education, careers, and independent living. These and other changes in the educational, social, and economic landscapes may have affected all youth, raising interest in how the characteristics, experiences, and challenges of youth with disabilities have changed over time. The National Longitudinal Transition Study (NLTS) 2012 provides updated information on youth with disabilities in light of these changes, to inform efforts to address their needs. Sponsored by the U.S. Department of Education under a congressional mandate to study IDEA 2004 and the students it serves, the NLTS 2012 is the third in a series of such studies. It describes the backgrounds of secondary school youth and their functional abilities, activities in school and with friends, academic supports received from schools and parents, and preparation for life after high school. Through surveys in 2012 and 2013, the study collected data on a nationally representative set of nearly 13,000 students--mostly those with an individualized education program (IEP) and expected to receive special education services. The study also includes students without an IEP, who either have no identified disability or who have an impairment that does not qualify them for special education but allows them to receive accommodations through a 504 plan under the Rehabilitation Act, another federal law pertaining to the rights and needs of youth with disabilities. This third volume of findings from the NLTS 2012 uses data from all three studies in the NLTS series to examine how the characteristics and experiences of youth in special education have changed over time, overall and for each of 12 disability groups defined by IDEA 2004. Most of the analyses examine trends for in-school youth ages 15 to 18 from 2003 to 2012, using the NLTS2 and NLTS 2012. When comparable data are available from the NLTS, the volume also examines trends starting in 1987 for youth ages 15 to 18 and youth ages 19 to 21 who were still enrolled in high school. The trends from 2003 to 2012 for youth with an individualized education program (IEP) ages 15 to 18 suggests several key points: (1) Youth with an IEP are more likely than a decade ago to live in households that face economic challenges; (2) Youth with an IEP are about as healthy and able to perform some typical tasks independently as in the past, but they are also more likely to use behavioral medicines and have trouble understanding others; (3) Engagement in school and extracurricular activities among youth with an IEP increased in the past decade, whereas the prevalence of negative events such as grade retention, suspensions, and expulsions was little changed; (4) Youth with an IEP are more likely than in the past to receive supports at school but less likely to get them at home; and (5) Participation in key transition activities by youth with an IEP and their parents has declined, although they are just as likely to have gone to an IEP meeting. [For "Preparing for Life after High School: The Characteristics and Experiences of Youth in Special Education. Findings from The National Longitudinal Transition Study 2012. Volume 3: Comparisons over Time. Executive Summary. NCEE 2018-4008," see ED580936. For "Preparing for Life after High School: The Characteristics and Experiences of Youth in Special Education. Findings from the National Longitudinal Transition Study 2012. Volume 2: Comparisons across Disability Groups. Full Report. NCEE 2017-4018," see ED573334.]