We argue that the input-output network of investment goods across sectors is an important propagation mechanism for understanding business cycles. First, we show that the empirical network is dominated by a few “investment hubs” that produce the majority of investment goods, are highly volatile, and are strongly correlated with the cycle. Second, we embed this network into a multisector model and show that shocks to investment hubs have large aggregate effects while shocks to non-hubs do not. Finally, we measure realized sector-level productivity shocks in the data, feed them into our model, and find that hub shocks account for a large and increasing share of aggregate fluctuations. This fact allows the model to match the decline in the cyclicality of labor productivity and other business cycle changes since the 1980s. Our model also implies that investment stimulus policies increase employment throughout the economy but have unequal effects across sectors.