As demonstrated by their emphasis in the new, national, science education standards, learning progressions (LPs) have become a valuable means of informing teaching and learning. LPs serve this role by isolating the key components of central skills and understandings, and by describing how those abilities and concepts tend to develop over time among students in a particular context. Some LPs also identify common challenges students experience in learning specific content and suggest methods of instruction and assessment, particularly ways in which difficulties can be identified and addressed. LPs are research-based and created through the integration of content analyses and interpretations of student performances with respect to the skills and understandings in question. The present research produced two LPs portraying the development of understandings associated with the second law of thermodynamics as evidenced by the evolving explanations for the spontaneity and irreversibility of diffusion and the cooling of a hot object constructed periodically by twenty students over two consecutive years in high school chemistry. While the curriculum they experienced did not emphasize the processes of diffusion and cooling or the second law and its applications, these students received prolonged instruction regarding key aspects of the particulate nature of matter. Working in small groups and as individuals, they were also taught and regularly expected to create, test, and revise particulate-based, conceptual models to account for the properties and behavior of a wide variety of common phenomena. Although some students quickly exhibited dramatic improvements in explaining and understanding the phenomena of interest, conceptual development for most was evolutionary rather than revolutionary, and success in explaining one phenomenon did not generally translate into successes in explaining related but different phenomena. Few students reached the uppermost learning goals of either LP, but the results of this study nevertheless provide valuable guidance in ways that the desired understandings can be cultivated in more students. These cognitive models of learning can provide teachers with valuable information capable of supporting improvements in instructional techniques and materials, assessment tools and methods, and perhaps even their own understandings of fundamental, scientific concepts. [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.]