With rapid technological transformations to the labor market, many working adults return to college after graduation to obtain additional training or credentials. Using a comparative individual fixed effects strategy and an administrative panel dataset of enrollment and employment in Virginia, we provide the first causal estimates of credential "stacking" -- earning two or more community college certificates or degrees -- among working adults. We find stacking increases employment by four percentage points and quarterly wages by $375 (four percent). Returns are larger for individuals studying in Health and who return to college after first completing a short-term certificate.