We study the degree of synchronization between formal- and informal-economy business cycles. Using a comprehensive database of informal activity that covers a wide range of informality measures from almost 160 countries over the 1990-2018 period, we report two major results. First, fluctuations in informal-sector output are strongly positively correlated with those in formal-sector output. In contrast, fluctuations in informal employment are largely uncorrelated with those in formal-sector output. Second, movements in the formal economy tend to spillover to the informal economy. Using a novel set of instrumental variables, we show that fluctuations in formal-sector output ＂cause＂ movements in informal-sector output.