We study productivity before and during the working from home [WFH] period of the Covid-19 pandemic, using personnel and analytics data from over 10,000 skilled professionals at a large Asian IT services company. Hours worked increased, including a rise of 18% outside normal business hours. Average output declined slightly, thus productivity fell 8-19%. We then analyze determinants of changes in productivity. Employees with children at home increased work hours more and had a larger decline in productivity than those without children. Women had a larger decline in productivity, while those with longer company tenure fared better. An important source of changes in WFH productivity is higher communication and coordination costs. Time spent on coordination activities and meetings increased, while uninterrupted work hours shrank considerably. Employees communicated with fewer individuals and business units, both inside and outside the firm. They also received less coaching and 1:1 meetings with supervisors. The findings suggest key issues for firms to address in implementing WFH policies.