There are at least four ways teachers may enter a new school: directly after receiving a new degree, exiting a different career, transferring from another school or type of position in a school, or after a break from teaching. The data used in this report span 25 years, from 1987 to 2012, providing an overview of these four key sources of newly hired teachers in the United States using the Schools and Staffing Survey (SASS). Four administrations of SASS (1987-88, 1999-2000, 2007-08, and 2011-12) provide data tracing the demographic characteristics, experience, qualifications, and prior year activities of newly hired K-12 teachers in the United States. Newly hired teachers are those who are new to teaching or those who are in a new position at a school in a different district or system in the academic year of the survey administration and taught at least half time or more at the school. The four sources of newly hired teachers described in this report follow: (1) Newly prepared teacher: first-year teacher (no previous experience) whose main activity in the prior year was attending college or who earned his or her highest degree in the prior year; (2) Delayed entrant: first-year teacher who in the prior year had engaged in other activities outside of attending college and teaching and had received the highest degree more than 1 year prior to the survey administration; (3) Transfer: teacher with previous teaching experience whose main activity in the prior year was working in another school outside of the current school system or another sector (public or private school); or (4) Re-entrant: teacher with previous experience whose main activity in the prior year was not teaching in grades K-12 but had taught in the past. Estimates are produced from cross-tabulations of the data, and Student's t tests are performed to test for differences between estimates. Only findings that met the a = 0.05 significance level are discussed in the text. The following are appended: (1) Standard Error Tables; (2) Technical Notes and Methodology; and (3) Glossary of Variables Used in This Report.