"This dissertation consists of three essays studying different aspects of international economics. The first two chapters focus on international trade, namely on estimating the size of trade barriers by looking at how firms manage their inventories, while Chapter 3 focus on international macroeconomics; in specific, the likelihood of a country to default on its debt when there is an informal sector. The first chapter provides evidence supporting the common assumption that international fixed ordering costs are higher than domestic fixed ordering costs. The canonical inventory model (the EOQ model) is extended to include two inputs sourced from different countries. Given the demand for each of the inputs, the fixed ordering costs and the inventory holding cost, the firm decides the optimal quantity to order each period. The model is estimated using firmlevel data on inventories of raw materials and inputs used from domestic and international sources. Assuming constant returns on inventory holding costs, the model reveals that it is between 20 and 60 times more expensive to place an order internationally than domestically, but yields an elasticity of inventories to demand much smaller than in the data. Allowing for a more general holding cost structure, that depends on the level of inventories in stock captures the variation of inventories' cost with firm's size. With this more general setup, foreign ordering costs are estimated to be between 3.5 and 5.2 times higher than domestic, suggesting that there are strong economies of scale in holding inventories. Those estimates are corroborated when I allow total fixed ordering costs to depend on total demand as this specification results in international fixed ordering costs between 4.1 and 7.2 times higher than domestic. The second chapter uses firm-level data on inventory holdings and source of inputs to estimate domestic and international trade barriers looking not only at fixed costs, but also at time lags and computing their tariff equivalents. It starts by documenting three features related to inventories, import decisions, and firm's size. First, inventories increase strongly in size, with an elasticity right below one. Second, importers hold more inventories than non-importers and third, inventories increase in import intensity. Given inventory carrying costs, the inventory holdings are used to infer relative domestic and international trade barriers. I develop a model of heterogeneous firms that produce using imperfectly substitutable domestic and imported intermediates and face demand and supply uncertainty. Given ordering costs and delivery lags that differ by source country, interest charges and inventory holding costs, producers use inventories to economize on trade costs. I find it is 5 times more costly to place an international than a domestic order but, when scaled by average shipment size, the international fixed ordering cost is just twice as large; the international time lag is 3 times larger than the domestic and there is complementarity in inputs, reflected in higher domestic inventories to domestic purchases ratio for importers than for non-importers. Overall, domestic and international trade frictions have a 17.3% tariff equivalent. I decompose these tariffs into their three components and observe that due to the substitutability between fixed ordering costs and inventory holding costs, these barriers have the same relevance while that of time lag is slightly smaller. This framework can then be used to evaluate the benefits of infrastructure investments and policy changes to reduce time delays, uncertainty or fixed ordering costs. The last chapter starts from the observation that, although emerging markets are often characterized by a large informal sector, frequent default and procyclical fiscal policies, sovereign default models proposing explanations for the high sovereign bonds interest rate spreads faced by developing economies have abstracted from the existence of the informal sector and its role. To address this concern, I propose a mechanism through which the size of the informal sector impacts a country's default decision. I extend a small open economy sovereign default model by including an informal and a formal sector and pro-cyclical fiscal policies, where a benevolent government makes default and tax decisions in order to maximize agent's utility and satisfy its level of public spending. I conclude that the taxable base decreases in the size of the informal sector leading to more distortions, which translate into higher tax rates, and more frequent defaults and that these results are magnified over the business cycle"--Pages vi-viii.