The present article presents a rubric we developed for assessing the quality of scientific explanations by science graduate students. The rubric was developed from a qualitative analysis of science graduate students' abilities to explain their own research to an audience of non-scientists. Our intention is that use of the rubric to characterise explanations of science by scientists, some of whom become professors, would lead to better teaching of science at the university level. This would, in turn, improve retention of qualified and diverse scientists, some of whom may elect to become science teachers. Our rubric is useful as an instrument to help evaluate scientific explanations because it distinguishes between the content knowledge and pedagogical knowledge of scientists, as well as a scientist's ability to integrate the two in the service of a clear and coherent explanation of his or her research. It is also generally useful in evaluating, or self-evaluating, science explanations by science professors and researchers, graduate students preparing to be scientists, science teachers and pre-service teachers, as well as students who are explaining science as part of learning. (Contains 5 tables.)