Throughout the series, demigods often have dreams of importance, indicating a message from someone else, or what may happen in the future.
Percy has several dreams of Kronos, trying to convince him to join the Titan army. Percy also had dreams of Thalia Grace, as well as one with a horse fighting an eagle, they represent Poseidon and Zeus respectively.
Percy has a dream of Grover in a wedding dress in a bridal shop, trying to run away, as he is coming. These dreams are revealed to be the result of an empathy link Grover created between them to communicate with Percy for help.
Percy dreams Daedalus and his son Icarus trapped in his workshop by King Minos. Icarus created wings to help himself escape, but did not listen to Daedalus' warning to not to go too far up in the air and he plunges to his death.
Percy soon afterwards dreams of Daedalus with his nephew Perdix. Daedalus kills Perdix by making him fall out the window. Athena sees this and punishes Daedalus by engraving a partridge into his skin.
Percy dreams of Maria di Angelo and Hades arguing about the safety of their children Bianca and Nico. Hades says that he should bring the kids to the Underworld, but Maria disagrees. Soon after, Maria is killed by Zeus' lightning bolt, and Hades immediately sends Nico and Bianca to the River Lethe then the Lotus Hotel and Casino.
Piper McLean dreams of the giants telling her to bring Jason Grace and Leo Valdez to Mount Diablo if she wants her father home safe. Lupa contacts Jason in his sleep to save the Wolf House. Hephaestus contacts Leo in his dreams but the transmission kept picking up Wheel of Fortune.
Percy's dreams are the only thing keeping him connected to his past at Camp Half-Blood. Annabeth appears to Percy in one dream, marking her only appearance in The Son of Neptune. She warns Percy to stay where he is, because Tyson and Mrs. O'Leary are coming to find him. Percy must leave however, because of the quest.
Percy's dreams about the Twins Giants, and the located and imprisonment of Nico di Angelo in Rome. He also has dreams with Gaea telling him that she would destroy Camp Half-Blood and Camp Jupiter once she has awoken, and that she would use him as her sacrifice along with Annabeth to reawaken her in August. Annabeth also has dreams of the Mark of Athena including about Arachne and the Statue.
Leo dreams that the Romans invasion of Camp Half-Blood ends in heavy loss on both sides and the giant army telling him he can't win. Jason dreams of Rachel's meeting with the Romans.
Jason has nightmares about his mother.
Hazel Levesque tells Apollo and Meg McCaffrey that she had a dream showing Jason’s death. Apollo dreams of Commodus and Caligula sailing north to the Bay Area and back to the early days of Rome when the The Oracle of Cumae and Tarquin were technically alive.
When Zane Obispo, his Uncle Hondo and Brooks first leave to find the hero twins, Zane fell asleep and had a dream. He was in a lush jungle with sharp trees made out of aluminum. His dog, Rosie, who was killed by a Demon Runner, appears and runs off. Zane runs after her, his short leg was same size as the other. Rosie turned into mist and disappeared, the trees reflected bad imitations of her. Wind caused the trees to make a screeching sound, the moon turned red and rain flooded the jungle. Zane rode a wave towards lightning strikes that burned the land, he felt his hands burning and yellow and red sparks jumped from his fingertips, heard Antonia Caballero tell him that he left her as a chicken.