Although young children are major audiences of science museums, limited evidence exists documenting changes in children's knowledge in these settings due in part to the limited number of valid and reliable assessment tools available for use with this population. The purposes of this study were to develop and validate a concept mapping assessment tool and to use this tool to document the butterfly-related knowledge of young children on unguided family visits to a live butterfly exhibit at a natural history museum. In this study, forty-two children visited the live butterfly exhibit with their families on unguided tours and completed pre- and post-visit concept mapping tasks. During pre- and post-visit mapping sessions, children created and revised concept maps about butterflies using a set of eight butterfly-related concept pictures and provided verbal explanations for each picture pair in their concept maps. Three raters used three different scoring systems designed for use in this study to evaluate the scientific accuracy of the children's pre- and post-visit maps, including the picture pairs and verbal explanations. Quantitative analyses of the scores indicate that the raters used the three scoring systems with a moderate to high level of consistency. The results also indicate that children significantly increased their butterfly-related knowledge in the live butterfly exhibit regardless of recent prior experience with the exhibit. Qualitative analyses of children's verbal explanations indicate that children possessed butterfly-related knowledge related to: the needs of butterflies, the life cycle of butterflies, the ecology of butterflies and other insects, the diversity and classification of butterflies and other insects, the threats to and conservation of butterflies, and the social and maternal behavior of butterflies. Although children with different levels of exhibit experience showed similarly high levels of prior and subsequent knowledge, the types of understandings they communicated in their verbal explanations differed somewhat. Surprisingly, children without recent prior exhibit experience showed greater understanding in more areas of butterfly-related knowledge than children with recent prior exhibit experience. The results of this study have implications for the field of science education in general and the field of informal science education in particular. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]