Caldwell's Case was a United States Supreme Court case that addressed the issue of whether a state could tax the income of a federal employee. The case was brought by William Caldwell, a federal employee who was being taxed by the state of Pennsylvania on his income. Caldwell argued that the state had no authority to tax his income, as it was derived from the federal government. The Supreme Court ultimately ruled in favor of Caldwell, holding that the state of Pennsylvania had no authority to tax the income of a federal employee. The Court reasoned that the federal government had exclusive authority over the income of its employees, and that the state could not interfere with this authority. The Court also noted that the federal government had the power to tax its own employees, and that the state could not interfere with this power. The decision in Caldwell's Case established the principle that states cannot tax the income of federal employees. This principle has been applied in numerous cases since then, and has been used to protect the rights of federal employees from state taxation.

In Caldwell's Case, the Supreme Court was tasked with determining whether a state law that imposed taxes on railroad companies violated the Fourteenth Amendment. The majority opinion held that it did not violate the amendment because it applied equally to all railroads and thus did not discriminate against any particular company or group of people. However, in his dissenting opinion Justice Field argued that while the tax may have been applied evenly across all railroads, its effect was still discriminatory as some companies were able to pay more than others due to their size and financial resources. He further argued that this type of taxation could lead to an unequal distribution of wealth among different classes of citizens which would be unconstitutional under the Fourteenth Amendment. Ultimately he concluded by stating that such laws should only be allowed if they are necessary for public safety or welfare and do not create inequality between individuals or groups based solely on their economic status.