This dissertation investigates the comparative affordances and drawbacks of blocks-based, text-based, and hybrid blocks/text introductory programming tools. Blocks-based programming environments are growing in popularity and are increasingly being used in formal introductory programming contexts. To date, much of the work evaluating such tools has focused on their effectiveness in out-of-school contexts and emphasized engagement and attitudinal measures over content mastery. Given their growing presence in classrooms, it is important to understand the benefits and limitations of the use of the blocks-based programming approach in formal learning contexts relative to text-based or hybrid blocks/text alternatives. This dissertation will carry out a quasi-experimental study in high school computer science classrooms to answer questions related to the impact of blocks-based, text-based, and hybrid blocks/text introductory tools, assess the suitability of such tools for preparing students for future computer science learning opportunities, and explore the design space between blocks-based and text-based programming. The goal of this work is to better understand the tools we are using to introduce today's learners to computer science and lay the foundation for creating the tools of tomorrow.