 Philosophical skepticism is interesting because there are intriguing arguments for it despite its initial implausibility. Many contemporary epistemological positions can be fruitfully presented as responding to some aspect of those arguments. For example, questions regarding principles of epistemic closure and transmission are closely related to the discussion of what we will call Cartesian Skepticism, as are views according to which we are entitled to dismiss skeptical hypotheses even though we do not have evidence against them. The traditional issue of the structure of knowledge and justification, engendering Foundationalism, Coherentism, and Infinitism, can be seen as resulting from one main argument for what we will call Pyrrhonian Skepticism. In what follows we present these two forms of skepticism and assess the main arguments for them.