The Cyrenaics or Kyrenaics (Ancient Greek: Κυρηναϊκοί; Kyrēnaïkoí) were a sensual hedonist Greek school of philosophy founded in the 4th century BCE, supposedly by Aristippus of Cyrene, although many of the principles of the school are believed to have been formalized by his grandson of the same name, Aristippus the Younger. The school was so called after Cyrene, the birthplace of Aristippus. It was one of the earliest Socratic schools. The Cyrenaics taught that the only intrinsic good is pleasure, which meant not just the absence of pain, (as it did for Epicurus) but positively enjoyable sensations. Of these, momentary pleasures, especially physical ones, are stronger than those of anticipation or memory. They did, however, recognize the value of social obligation and that pleasure could be gained from altruistic behaviour. The school died out within a century and was replaced by the philosophy of Epicureanism.
The history of the Cyrenaic school begins with Aristippus of Cyrene, who was born around 435 BCE. He came to Athens as a young man and became a pupil of Socrates. We have only limited knowledge of his movements after the execution of Socrates in 399 BCE, although he is said to have lived for a time in the court of Dionysius of Syracuse.
It is uncertain precisely which doctrines ascribed to the Cyrenaic school were formulated by Aristippus.Diogenes Laërtius, based on the authority of Sotion and Panaetius, provided a long list of books said to have been written by Aristippus. However, Diogenes also wrote that Sosicrates had stated that Aristippus had written nothing. Among Aristippus' pupils was his daughter, Arete of Cyrene, who passed on his teachings to her own son Aristippus the Younger. It was he, according to , who turned the teachings of his grandfather into a comprehensive system. At the least, however, it can be said that the foundations of Cyrenaic philosophy were ideas originated by the elder Aristippus.