The field is generally aware of summer learning loss (SLL)--that student learning slows during the summer. Yet surprisingly little consensus exists on basic questions about SLL, especially given recent concerns about measurement artifacts in foundational SLL studies. Nearly all prior SLL work examines how summers contribute to racial/ethnic or socioeconomic gaps. However, these factors only account for about 4 percent of the variance in summer learning rates. We use a unique dataset with millions of students across eight grades to document the full spread of SLL and examine how differential SLL contributes to where students end up in the 8th grade achievement distribution. We find dramatic variability in SLL, with decrements accruing to the same students over time. [Additional funding provided by the Kingsbury Center at the Northwest Evaluation Association (NWEA).]