In an attempt to further understand connections between children's proficiency and development with single- and multidigit addition, this study investigated the conceptualizations and solution strategies of 26 first-graders presented with several single-digit, multiple addend addition problems. The changes in students' solution strategies over the course of an academic year showed that student development was progressive, as with single-digit addition, but variable and contextual, much like multidigit addition. Students generally moved from counting strategies in the Fall, to various modeling strategies in the Spring, but did not consistently apply one strategy over another and often used different strategies for different sums or addends. The use of multiple addend addition problems forced students to move beyond fact mastery and blend strategies to create original approaches to novel problem situations. Such variability in solution strategies promotes the development of rich intermediary stages and provides a powerful opportunity for teachers and students to use, develop, share, and discuss various problem-solving approaches and conceptual understandings. (Contains 2 figures and 6 tables.)