This thesis consists of four essays on the role of firm heterogeneity for monetary policy transmission. Firm Heterogeneity and Monetary Policy Transmission investigates which firm characteristics make firm investments more sensitive to monetary policy and highlights the potential role of investment frictions for a stronger transmission to young firms. Monetary Policy Transmission in the Presence of Investment Frictions analyzes investment dynamics and responses to interest rate changes in a model with capital adjustment costs. Heterogeneity in Corporate Debt Structures and the Transmission of Monetary Policy studies how differences in the aggregate structure of corporate debt - bank loans and corporate bonds - affect the transmission of monetary policy. Corporate Leverage and Monetary Policy Transmission examines how the macroeconomic transmission of monetary policy changes when there are unexpected shifts in aggregate corporate leverage. This is a doctoral thesis in Economics at Stockholm University, Sweden 2023