This paper proposes that idiosyncratic firm-level fluctuations can explain a significant portion of aggregate shocks and provide a microfoundation for aggregate productivity shocks. The author challenges the traditional view that individual firm shocks average out in aggregate, arguing that this argument breaks down when the distribution of firm sizes is fat-tailed, as empirically documented. The study finds that the idiosyncratic movements of the largest 100 firms in the US account for about one-third of variations in output and the Solow residual. This "granular" hypothesis suggests new directions for macroeconomic research, particularly that macroeconomic questions can be clarified by examining the behavior of large firms. The paper also discusses the implications for other economic aggregates, such as exports or the trade balance. The author uses a simple model to illustrate the idea and provides a richer model to measure idiosyncratic shocks and explain production linkages. Empirical evidence supports the hypothesis, showing that idiosyncratic shocks to large firms are of the correct order of magnitude to explain business cycles. The paper concludes by discussing the implications for the literature on sector-specific shocks and aggregate shocks, and suggests that the fat-tailed nature of firm size distributions is important for understanding aggregate volatility.This paper proposes that idiosyncratic firm-level fluctuations can explain a significant portion of aggregate shocks and provide a microfoundation for aggregate productivity shocks. The author challenges the traditional view that individual firm shocks average out in aggregate, arguing that this argument breaks down when the distribution of firm sizes is fat-tailed, as empirically documented. The study finds that the idiosyncratic movements of the largest 100 firms in the US account for about one-third of variations in output and the Solow residual. This "granular" hypothesis suggests new directions for macroeconomic research, particularly that macroeconomic questions can be clarified by examining the behavior of large firms. The paper also discusses the implications for other economic aggregates, such as exports or the trade balance. The author uses a simple model to illustrate the idea and provides a richer model to measure idiosyncratic shocks and explain production linkages. Empirical evidence supports the hypothesis, showing that idiosyncratic shocks to large firms are of the correct order of magnitude to explain business cycles. The paper concludes by discussing the implications for the literature on sector-specific shocks and aggregate shocks, and suggests that the fat-tailed nature of firm size distributions is important for understanding aggregate volatility.