This NetLogo simulation, developed by Uri Wilensky at Northwestern University, models three populations (wolf, sheep, grass) within an ecosystem over time. Settings within the simulation help students to make sense of the phenomenon of carrying capacity by allowing them to mimic changes in initial population sizes, reproduction rates, and energy gain from food. By manipulating these settings, students can ask questions and then look for evidence to support their thinking about what factors affect carrying capacity and cause populations to be either stable or unstable over time. Instructional supports include background information, instructions for using the simulation, and suggestions for extending the simulation. In addition, two papers written by Wilensky are referenced. One is available online at: http://ccl.northwestern.edu/papers/wolfsheep.pdf.