The purpose of this research study was to examine the differences in beliefs and perceptions about the implementation of constructivist principles into instruction, in support of the National Science Education Standards, for science teachers who adopt constructivist principles and those who do not. The study also examined correlations between a teacher's years of experience and his/her implementation of constructivist principles in instruction. The sample included teachers from all over the United States who were members of the National Science Teachers Association and also used the National Science Education Standards in their instruction. All demographic data was descriptively summarized. The two groups were compared on their overall constructivist principles survey score using the dependent sample t-test, and the results were significant (p less than or equal to 0.05). The data revealed no significant differences between teachers who implement constructivist practices and those who do not with regard to their beliefs and perceptions. There was no significant difference found between teachers who implement constructivist practices and those who do not with regard to their scientific knowledge mean rating, or their constructivist practices rating (p greater than 0.05). [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]