An acute shortage of a competent, highly-skilled workforce faces the United States workplace. Studies and reports from 1983 to present, repeatedly state that the education system in the United States must change to prepare the emergent workforce for success in the 21st century global challenges of both post-secondary education and the workplace. The rising high school drop-out rate and the Millennial generation's lack of critical workplace skills concern the U.S. employers who must successfully compete globally within a shrinking economy. This study tests the following hypothesis, H[subscript o], there is no difference in knowledge and understanding of the importance of 21st century skills between the treatment group of teachers who have experienced an educator externship and the control group of teachers who have not, against the alternate hypothesis, H[subscript 1], which states there is a difference in knowledge and understanding of the importance of 21st century skills between teachers who experience the educator externship and those not participating in an externship. A mixed methods static group comparison research design was used in the development of a 48 question survey to collect quantitative data. The survey was electronically administered to the control group N = 44 of teachers who have not experienced an externship and to the treatment group of teachers N = 33 who have completed an externship. The data collected were analyzed using a one way ANOVA to determine rejection of the null hypothesis. The qualitative data used in this study were gathered from the four open-ended questions on the 21st century skills survey directed to the teachers who had completed an externship; the data were examined using the "zig-zag" approach (Creswell, 2002) where categories, themes, and patterns emerged. Directors of educator externship programs N = 4 yielded information about any pre and post professional development administered to teachers regarding the 21st Century Skills. The results of the study failed to reject the alternate hypothesis and validated the externship as an authentic professional development option that provides knowledge of the 21st Century Skills for teachers, which supports the millennial students' requests for applied learning activities which will be meaningful and relevant to their lives. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]