Although long walls were built at several locations in ancient Greece, notably Corinth and Megara, the term Long Walls (Ancient Greek: Μακρὰ Τείχη [makra tei̯kʰɛː]) generally refers to the walls that connected Athens' main city to its ports at Piraeus and Phaleron.

Built in several phases, they provided a secure connection to the sea even during times of siege. The walls were about 6 km (3.7 mi) in length. They were initially constructed in the mid-5th century BC, and destroyed by the Spartans in 403 BC after Athens' defeat in the Peloponnesian War. They were rebuilt with Persian support during the Corinthian War in 395–391 BC.

The Long Walls were a key element of Athenian military strategy, since they provided the city with a constant link to the sea and thwarted sieges conducted by land alone.

The ancient wall around the Acropolis was destroyed by the Persians during the occupations of Attica in 480 and 479 BC, part of the Greco-Persian Wars. After the Battle of Plataea, the invading Persian forces were removed and the Athenians were free to reoccupy their land and begin rebuilding their city. Early in the process of rebuilding, construction started on new walls around the city proper.

This project drew opposition from the Spartans and their Peloponnesian allies, who were alarmed by the recent increase in the power of Athens. Spartan envoys urged the Athenians not to go through with the construction, arguing that a walled Athens would be a useful base for an invading army, and that the defenses of the Isthmus of Corinth would provide a sufficient shield against invaders.

However, despite these concerns, the envoys did not strongly protest and in fact gave helpful advice to the builders. The Athenians disregarded their negative arguments, fully aware that leaving their city unwalled would place them utterly at the mercy of the Peloponnesians; Thucydides, in his account of these events, describes a series of complex machinations by Themistocles through which he distracted and delayed the Spartans until the walls were built up high enough to provide adequate protection.

In the early 450s BC, fighting began between Athens and various Peloponnesian allies of Sparta, particularly Corinth and Aegina. In the midst of this fighting between 462 BC and 458 BC, Athens had begun construction of two more walls, the Long Walls, one running from the city to the old port at Phalerum, the other to the newer port at Piraeus. In 457 BC, a Spartan army defeated an Athenian army at Tanagra while attempting to prevent the construction, but work on the walls continued and they were completed soon after the battle. These walls ensured that Athens would never be cut off from supplies as long as she controlled the sea. These Phase 1a walls enclosed a vast area and included Athens' two main ports.

The building of the Long Walls reflected a larger strategy that Athens had come to follow in the early 5th century. Unlike most Greek city states, which specialized in fielding Hoplite armies, Athens had focused on the navy as the centre of its military since the time of the building of her first fleet during a war with Aegina in the 480s BC.