Using firm-level data on listed non-financial companies in 14 advanced economies, we document a rise in the share of zombie firms, defined as unprofitable firms with low stock market valuation, from 4% in the late 1980s to 15% in 2017. These zombie firms are smaller, less productive, more leveraged and invest less in physical and intangible capital. Their performance deteriorates several years before zombification and remains significantly poorer than that of non-zombie firms in subsequent years. Over time, some 25% of zombie companies exited the market, while 60% exited from zombie status. However, recovered zombies underperform compared to firms that have never been zombies and they face a high probability of relapsing into zombie status.