This article provides quasi-experimental evidence on the relationship between employee happiness and productivity in the field. We study the universe of call center sales workers at British Telecom (BT), one of the United Kingdom's largest private employers. We measure their happiness over a 6 month period using a novel weekly survey instrument, and link these reports with highly detailed administrative data on workplace behaviors and various measures of employee performance. Exploiting exogenous variation in employee happiness arising from weather shocks local to each of the 11 call centers, we document a strong causal effect of worker happiness on sales. This is driven by employees working more effectively on the intensive margin by makin...