My dissertation explores various topics in macroeconomics related to the level of aggregate income in different countries and how (un-)equally it is distributed across people within a country. More specifically, I focus on firms: who owns them, how they are financed, and how their production processes connect them to other sectors of the economy. In the first chapter, I study how financial markets affect the distribution of wealth across households through their effect on ownership structures of firms. I show that, within Europe, there are countries in which firms are broadly owned and financed by large parts of the population. In these countries, business risk is more widely spread across people, and wealth is less concentrated in the hands of just a few households. The remaining two chapters are concerned with the specific challenges facing firms in developing countries. I study the interaction of different sectors of the economy and what the nature of interlinkages implies for the size of firms and aggregate productivity. These are first-order issues when thinking about policies to close the gap in output and productivity between developing and developed countries. In the first chapter, `Owning Up: Closely Held Firms and Wealth Inequality', I study how frictions in debt and equity markets affect wealth inequality in Eurozone countries. Using micro data on households and firms, I document that in more unequal countries, there are more privately held firms, and ownership of publicly traded firms is more concentrated. I develop a dynamic general equilibrium model in which entrepreneurs have the option to run a private firm and issue debt, or go public and also issue outside equity. Both forms of external finance are subject to country-specific frictions. With more access to debt, entrepreneurs can run larger firms and are wealthier. Similar to debt, outside equity allows entrepreneurs to increase investment in their firm, but it also reduces their risk exposure, which lowers savings and wealth holdings. When parameters are chosen to match the facts I document on firm ownership and financing, the model successfully predicts differences in wealth concentration across countries. The second chapter, `The Aggregate Importance of Intermediate Input Substitutability', is co-authored with Stanford PhD graduate Cian Ruane. In this chapter we ask whether economic development policies should target specific sectors of the economy or follow a `big push' approach of advancing all sectors together. The relative success of these strategies is determined by how easily firms can substitute between intermediate inputs sourced from different sectors of the economy: a low degree of substitutability increases the costs from `bottleneck' sectors and the need for `big push' policies. We estimate long-run elasticities of substitution between intermediate inputs used by Indian manufacturing plants. We use detailed data on plant-level intermediate input expenditures, and exploit reductions in import tariffs as plausibly exogenous shocks to domestic intermediate input prices. We find a long-run plant-level elasticity of substitution of 4.3, a much higher level of substitutability than existing short-run estimates or the Cobb-Douglas benchmark. To quantify the aggregate importance of intermediate input substitution, we embed our elasticities in a general equilibrium model with heterogeneous firms, calibrated to plant- and sector-level data for the Indian economy. We find that the aggregate gains from a 50% productivity increase in any one Indian manufacturing sector are on average 47% larger with our estimated elasticities. Our counterfactual exercises highlight the importance of intermediate input substitution in amplifying policy reforms targeting individual sectors. The third chapter, `Distribution Costs and the Size of Indian Manufacturing Establishments', is also co-authored with Cian Ruane. We explore how productivity improvements in the distribution sectors of the Indian economy, such as transportation or wholesale trade, impact firms in the manufacturing sector. The sale of manufacturing goods involves costs of distribution such as shipping, insurance and commissions. Using micro-data from India's Annual Survey of Industries, we document that larger plants spend a larger share of their sales on distribution. We ask to what degree these distribution costs act as a constraint on larger firms and can explain the high employment share in small plants. To explore this mechanism, we develop a simple general equilibrium model in which heterogeneous firms face fixed and variable costs of distributing their products to customers. Firms selling higher quality products sell to more distant customers and incur higher distribution expenses. We carry out some preliminary quantitative exercises to explore how much TFP increases in the distribution sector affect aggregate consumption and the firm size distribution.