In this dissertation we study the sources of aggregate fluctuations with emphasis on the role of firms' linkages and non-linearities in production. In Chapters 1 and 2, we present a new set of empirical facts on i) the relationship between the structure of inter-sectoral linkages and aggregate fluctuations, and ii) the relationship between firms' elasticity of substitution in production and the spreads on corporate bonds during periods of high corporate debt and in recessions. We then construct a multisector model that is able to deliver the observed empirical patterns. In Chapter 3 we test the predictions of different dynamic multisector models regarding how well they predict the observed co-movement of sectoral output growth in the U.S. industrial sectors. Finally, in Chapter 4 we study the normative implications of combining the models in Chapters 1-3.In Chapter 1, we study what features of the countries' domestic structure of sectoral linkages can help account for the observed differences in cross-country volatility of growth and the size of macroeconomic downturns. The empirical cross-country evidence, crosssectional and panel data, suggests that unlike previous multisector models predict, the density of sectoral connections -number of sectoral connections in countries' input-output tables -is strongly correlated with aggregate volatility and the size of downturns. Therefore, we develop a multisector model that is able to deliver a role for the density of connections.The key implications of the model are that: i) the density of sectoral connections amplifies shocks -as observed for manufacturing oriented countries -if firms have low elasticity of substitution between labor and intermediates, while ii) density mitigates shocks -as observed for service oriented countries -if firms have high elasticity. In addition, iii) the density of connections amplifies the size of downturns -as observed for manufacturing oriented countries -if low elasticity firms face frictions in the use of intermediates, while iv) density mitigates downturns -as observed for service oriented countries -if high elasticity firms face frictions in the use of labor.In Chapter 2, we study what are the sectors of the economy that have more troubles in financing inputs during recessions in the U.S. Using firm level and sector level data, we find that during times of high corporate debt i) manufacturing sectors with lower estimated elasticities of substitution between labor and intermediates pay higher spread on corporate bonds, while service sectors with larger elasticities pay higher spreads on corporate bonds.We also find that during recessions firms with low elasticity between labor and interme- In Chapter 3, we contrast the predictions of different multisector DSGE models with the observed correlations of sectoral output growth in the U.S. industrial sectors. We study models that differ in the timing of the investment decision and in the technology used to build physical capital. To focus on the role of capital accumulation, we as...