Many teachers bring caterpillars into their class for their students to see the transformation from caterpillar to butterfly. This is exciting and educational, but more can be done using Monarchs across the curriculum. This article describes an integrated butterfly unit for students in grades four to six that starts with identifying the Monarch host plant (milkweed) and extends to include math, language arts, and social studies. A Monarch project has numerous opportunities for performance-based assessments, as students identify milkweed and Monarch eggs and caterpillars; care for the Monarchs; measure larvae, pupae, and butterflies and construct data tables and line graphs; and observe and record significant events in the Monarch's life cycle. (Contains 8 resources and 5 online resources.)