Stella II helps students understand the hows and whys underlying biological processes ranging from cellular respiration to ecosystem dynamics and genetic evolution. Stella II provides a set of simple building blocks that can be used to piece together the relationships that govern a biological system. This evaluation kit provides software tools that foster critical thinking skills in biology students through 23 simulations that allow students to control the learning process as they build a model, incorporate assumptions into their model, and then discover the dynamic implications of their assumptions. Alternatively, the instructor can lead the class in the construction of a model, providing students with immediate feedback based on their assumptions. Stella II's exceptional sensitivity analysis capabilities enable students and instructors to explore a full range of variation of key variables within the system under investigation.