In a series of research briefs, the Houston Education Research Consortium (HERC) examines and describes the amount of student mobility in Texas with particular focus on Houston area public schools. This first research brief provides an overview of how many students move during the summer months. Changing schools impacts students' achievement, educational attainment, and their relationships with peers and teachers. Mobile students tend to have lower grades and test scores, experience grade retention more frequently, and are more likely to drop out of school (Rumberger, 2003; South, Haynie, & Bose, 2007). As the evidence of student mobility's negative consequences grows, understanding the influence of mobility on schooling in Texas and the Houston area becomes increasingly important. Before examining mobility's impact, however, an understanding of its prevalence must be acquired. This research brief offers an initial, descriptive look at summer mobility, or mobility that takes place between school years. [For "Changing Schools, Part 2: Student Mobility during the School Year in Texas and the Houston Area. Research Brief for the Houston Independent School District. Volume 8, Issue 5," see ED611143.]