In the past ten years, increasing evidence has demonstrated that scientific teaching and active learning improve student retention and learning gains in the sciences. Graduate teaching assistants (GTAs), who play an important role in undergraduate education at many universities, require training in these methods to encourage implementation, long-term adoption, and advocacy. Here, we describe the design and evaluation of a two-day training workshop for first-year GTAs in the life sciences. This workshop combines instruction in current research and theory supporting teaching science through active learning as well as opportunities for participants to practice teaching and receive feedback from peers and mentors. Postworkshop assessments indicated that GTA participants' knowledge of key topics increased during the workshop. In follow-up evaluations, participants reported that the workshop helped them prepare for teaching. This workshop design can easily be adapted to a wide range of science disciplines. Overall, the workshop prepares graduate students to engage, include, and support undergraduates from a variety of backgrounds when teaching in the sciences.