Place-value is an essential concept that is foundational for understanding our number system and for developing procedures for multidigit operations, but there has been limited research into the place-value understanding of students with learning disabilities. This research is a two-part mixed-methods study that links basic research to instruction. The first part of the study consists of clinical interviews investigating how fifteen students with learning disabilities understand place-value in a variety of contexts. The second part focuses on teaching base-ten numeration to four students who demonstrated difficulties with this concept in the assessment study. Findings from the assessment study found that students with learning disabilities are a heterogeneous group, with five distinct profiles of place-value understanding emerging from the fifteen students. One of the common difficulties that the students had with place-value was difficulty understanding base-ten numeration, the concept of unitizing by tens, and this impacted their ability to solve mathematical word problems correctly. Findings from the instructional study found that counting collections (Schwedtfeger & Chan, 2007) was an effective instructional activity for improving students' understanding of base-ten numeration, as measured by accuracy and problem-solving strategies. This study argues that researchers should focus more on place-value understanding for students with learning disabilities, and proposes both an assessment to measure this understanding and an instructional activity to improve students' understanding of one facet of place-value understanding. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]