This paper examines the relation between children's summer activities before fourth through sixth grade and their vocabulary knowledge in fifth grade and at age fifteen using the NICHD SECCYD dataset (N = 1,009). We used OLS regression and propensity score analyses to understand how children's summer reading, library visits, participation in enrichment classes, and unsupervised time predicts their vocabulary knowledge. Propensity score matching and OLS analyses show that time spent reading predicts vocabulary during the following two years, and high levels of time allocated to reading across three or more summers in middle childhood predicts vocabulary knowledge at age 15. OLS analyses suggest a relationship between library visits and vocabulary knowledge. There is no short-term relationship between enrichment classes and vocabulary knowledge, although our OLS analysis demonstrated that consistent enrollment in summer enrichment classes over three years predicted improved vocabulary. Unsupervised time predicted poor vocabulary in both the short and long-term.