Since first being introduced in the mid 1990s, the term "citizen science"-the intentional engagement of the public in scientific research-has seen phenomenal growth as measured by the number of projects developed, people involved, and articles published. In addition to contributing to scientific knowledge, many citizen science projects attempt to achieve learning outcomes among their participants, however, little guidance is available for practitioners regarding the types of learning that can be supported through citizen science or the measuring of learning outcomes. This study provides empirical data to understand how intended learning outcomes first described by the informal science education field have been employed and measured within the citizen science field. We also present a framework for describing learning outcomes that should help citizen science practitioners, researchers, and evaluators in designing projects and in studying and evaluating their impacts. This is a first step in building evaluation capacity across the field of citizen science. Knowledge, Awareness, Understanding: Measurable demonstration of assessment of, change in, or exercise of awareness, knowledge, understanding of a particular scientific topic, concept, phenomena, theory, or careers central to the project. Strand (2), Understanding: Come to generate, understand, remember, and use concepts, explanations, arguments, models, and facts related to science. Engagement, interest or motivation in science: Measurable demonstration of assessment of, change in, or exercise of engagement/interest in a particular scientific topic, concept, phenomena, theory, or careers central to the project. Strand (1), Interest and motivation: Experience excitement, interest and motivation to learn about phenomena in the natural and physical world. Skills related to science inquiry: Measurable demonstration of the development and/or reinforcement of skills, either entirely new ones or the reinforcement, even practice, of developing skills. Strand (3), Science Exploration: Manipulate, test, explore, predict, question, and make sense of the natural and physical world; and Strand (5): Participate in scientific activities and learning practices with others, using scientific language and tools Attitudes toward science: Measurable demonstration of assessment of, change in, or exercise of attitude toward a particular scientific topic, concept, phenomena, theory, or careers central to the project or one's capabilities relative to these areas. Attitudes refer to changes in relatively stable, more intractable constructs such as empathy for animals and their habitats, appreciation for the role of scientists in society or attitudes toward stem cell research. Related to Strand (6), Identity: Think about themselves as science learners, and develop an identity as someone who knows about, uses, and sometimes contributes to science. Also, related to Strand (4), Reflection: Reflect on science as a way of knowing; on processes, concepts, and institutions of science; and on t...