This study used a modified mixed-method sequential explanatory design to explore how preschool teachers perceive and use complex talk within different activity settings and content areas to facilitate language and literacy skills. The combination of large-scale, quantitative data (455 children, 63 teachers) from a pre-K study and in-depth, qualitative interview data from a separate sample of 10 preschool teachers in rural North Carolina provided a holistic analysis of the phenomena of complex teacher talk. Major findings in the current study revealed teachers modulated their use of complex talk depending on instructional context and learner variability. The analysis of quantitative and qualitative data showed: (1) Teachers' use of complex talk was higher for large group than for small group and free choice/center time. (2) Complex teacher talk was higher for social studies and science than for math. (3) Teachers' use of complex talk in large-group, literacy, and math activities was positively related to their emotional support. (4) The use of complex talk in large-group activities was positively related to teaching experience. (5) Black children and dual language learners experienced lower levels of complex talk in small-group activities. (6) The use of complex talk in free play and social studies was positively related to children's shyness. Additionally, the analysis of quantitative data revealed larger gains in picture vocabulary were positively associated with the use of complex talk in large-group and science activities, indicating that the use of complex language in the activity in which the proportion of using complex talk was higher related to gains in children's language skills. Also, the analysis of quantitative data demonstrated the use of complex talk within different activity contexts was not related to teachers' attitude toward ECE and instructional orientation, while teachers qualitatively described the value of using complex talk in their classrooms. The combination of the quantitative and qualitative analysis illustrated the complexity of teachers' use of complex language across different activity contexts and expanded our understanding about the complex talk phenomena. Potential implications of these findings for ECE research, practice, and policy will be discussed. [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.]