This study sought to explore qualitatively how participation in an informal science program might affect the following aspects of upper elementary school children's scientific thinking: conceptual understanding, epistemology of science, and the formation of their identity as science learners. A purposefully selected, maximum variation sample of five upper elementary school children who had participated in an out-of-school (OST) science program was compared with five similarly selected upper elementary school children who had not participated in an OST science program. Semi-structured interviewing was the method of data collection. Findings reveal that upper elementary children exhibit some qualitative differences with respect to their conceptual understanding, epistemology of science, and formation of identity as science learners. In general, OST participants had more advanced (sophisticated) epistemologies of science than non-OST participants; OST participants also appeared to form stronger identities as science learners than non-OST participants. With respect to conceptual understanding, OST participants demonstrated greater understanding than non-OST participants of the conservation of matter, the physical properties of matter, and the composition of matter. Neither group had a clear understanding of the concepts of the density of various liquids and density as it relates to how objects made of different materials float. The findings from this study also indicate that there are qualitative differences in the in-school science experiences of upper elementary children exposed to OST settings and those not so exposed. OST participants were more able to rapidly recall their in-school science experiences than non-OST participants. OST participants were also able to transfer their OST science knowledge to their in-school science experiences. The theoretical perspectives employed in this study shed new light on the ways in which OST science experiences might impact children's science learning. [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.]