 Parmenides of Elea, active in the earlier part of the 5th c. BCE, authored a difficult metaphysical poem that has earned him a reputation as early Greek philosophy’s most profound and challenging thinker. His philosophical stance has typically been understood as at once extremely paradoxical and yet crucial for the broader development of Greek natural philosophy and metaphysics. He has been seen as a metaphysical monist (of one stripe or another) who so challenged the naïve cosmological theories of his predecessors that his major successors among the Presocratics were all driven to develop more sophisticated physical theories in response to his arguments. The difficulties involved in the interpretation of his poem have resulted in disagreement about many fundamental questions concerning his philosophical views, such as: whether he actually was a monist and, if so, what kind of monist he was; whether his system reflects a critical attitude toward earlier thinkers such as the Milesians, Pythagoreans, and Heraclitus, or whether he was motivated simply by more strictly logical concerns, such as the paradox of negative existentials that Bertrand Russell detected at the heart of his thought; whether he considered the world of our everyday awareness, with its vast population of entities changing and affecting one another in all manner of ways, to be simply an illusion, and thus whether the lengthy cosmological portion of his poem represented a genuine attempt to understand this world at all. This entry aims to provide an overview of Parmenides’ work and of some of the major interpretive approaches advanced over the past few decades. It concludes by suggesting that understanding his thought and his place in the development of early Greek philosophy requires taking due account of the fundamental modal distinctions that he was the first to articulate and explore with any precision.