In Greek mythology, Caerus /ˈsɪərəs, ˈsiːrəs/ (Greek: Καιρός, Kairos, the same as kairos) was the personification of opportunity, luck and favorable moments. He was shown with only one lock of hair. His Roman equivalent was Occasio or Tempus. Caerus was the youngest son of Zeus.

Caerus is the due measure that achieves the aim. This god brings about what is convenient, fit, and comes in the right moment. Sometimes it could be the critical or dangerous moment, but more often Caerus represents the advantageous, or favorable occasion. Hence, what is opportune, or "Opportunity." In the Hellenistic age (as P. Chantraine informs us), the term was also used as "time" or "season" (the good time, or good season).

According to Pausanias, there was an altar of Caerus close to the entrance to the stadium at Olympia, for Opportunity is regarded as a divinity and not as a mere allegory. This indefatigable traveler also tells us that Caerus was regarded as the youngest child of Zeus in a hymn by Ion of Chios (ca. 490-425 BC).

Caerus is represented as a young and beautiful god. Opportunity obviously never gets old, and beauty is always opportune, flourishing in its own season. Caerus stands on tiptoe because he is always running, and like Hermes, he has wings in his feet to fly with the wind. He holds a razor, or else scales balanced on a sharp edge—attributes illustrating the fleeting instant in which occasions appear and disappear. A. Fairbanks (translator of Callistratus) suggests that the type of the statue of Opportunity was developed out of the form of the Hermes that granted victory in athletic contests.

Caerus can easily be seized by the hair hanging over his face ("creeping down over the eyebrows") when he is arriving. But once he has passed by, no one can grasp him, the back of his head being bald. The moment of action is gone with his hair: a neglected occasion cannot be recovered. The author of Ekphráseis (Descriptions) found that the statue of Caerus at Sicyon resembled Dionysus, with his forehead glistening with graces and a delicate blush on his cheeks: "... though it was bronze, it blushed; and though it was hard by nature, it melted into softness". And like the statue is Opportunity himself, he melts into softness if caught by the forelock, but once he has raced by, he assumes his hard nature and seldom grants a second chance.

According to ancient Greeks, Kairos was the god of the "fleeting moment"; "a favorable opportunity opposing the fate of man". Such a moment must be grasped (by the tuft of hair on the personified forehead of the fleeting opportunity); otherwise the moment is gone and can not be re-captured (personified by the back of head being bald).

A bronze statue of Kairos is known in literature, made by the famous Greek sculptor Lysippos. It stood at his home, in the Agora of Hellenistic Sikyon. The following epigram by Poseidippos was carved on the statue:

Who and whence was the sculptor? From Sikyon.
And his name? Lysippos.
And who are you? Time who subdues all things.
Why do you stand on tip-toe? I am ever running.
And why you have a pair of wings on your feet? I fly with the wind.
And why do you hold a razor in your right hand? As a sign to men that I am sharper than any sharp edge.
And why does your hair hang over your face? For him who meets me to take me by the forelock.
And why, in Heaven's name, is the back of your head bald? Because none whom I have once raced by on my winged feet will now, though he wishes it sore, take hold of me from behind.