Policy documents in science education suggest that even at the earliest years of formal schooling, students are capable of constructing scientific explanations about focal content. Nonetheless, few research studies provide insights into how to effectively provide scaffolds appropriate for late elementary-age students' fruitful creation of scientific explanations. This article describes two research studies to address the question, what makes explanation construction difficult for elementary students? The studies were conducted in urban fourth, fifth, and sixth grade classrooms where students were learning science through curricular units that contained 8 weeks of scaffold-rich activities focused on explanation construction. The first study focused on the kind and amount of information scaffold-rich assessments provided about young students' abilities to construct explanations under a range of scaffold conditions. Results demonstrated that fifth and sixth grade tests provided strong information about a range of students' abilities to construct explanations under a range of supported conditions. On balance, the fourth grade test did not provide as much information, nor was this test curricular-sensitive. The second study provided information on pre-post test achievement relative to the amount of curricular intervention utilized over the 8-week time period with each cohort. Results demonstrated that when taking the amount of the intervention into account, there were strong learning gains in all three grade-level cohorts. In conjunction with the pre-post study, a type-of-error analysis was conducted to better understand the nature of errors among younger students. This analysis revealed that our youngest students generated the most incomplete responses and struggled in particular ways with generating valid evidence. Conclusions emphasize the synergistic value of research studies on scaffold-rich assessments, curricular scaffolds, and teacher guidance toward a more complete understanding of how to support young students' explanation construction. (Contains 11 figures, 7 tables and 2 notes.)