In this study we analyze the quality of students' written scientific explanations in eight science inquiry-based middle-school classrooms and explore the link between the quality of students' scientific explanations and their students' performance. We analyzed explanations based on three components: claim, evidence to support it, and a reasoning that justifies the link between the claim and evidence. Quality of explanations was linked with students' performance in different types of assessments focusing on the content of the science unit studied. To identify critical features related with high quality explanations we also analyzed the characteristics of the instructional prompts that teachers used. Results indicated that: (a) Students' written explanations can be reliably scored with the proposed approach. (b) The instructional practice of constructing explanations has not been widely implemented despite its significance in the context of inquiry-based science instruction. (c) Overall, a low percentage of students (18%) provided explanations with the three expected components. The majority (40%) of the "explanations" found were presented as claims without any supporting data or reasoning. (d) The magnitude of the correlations between students' quality of explanations and their performance, all positive but of varied magnitude according to the type of assessment, indicate that engaging students in the construction of high quality explanations might be related to higher levels of student performance. The opportunities to construct explanations, however, seem to be limited. We report some general characteristics of instructional prompts that showed higher quality of written explanations. (Contains 15 tables, 4 figures, and 5 footnotes.)