HH

HH may refer to:

People

• Helen Hunt Jackson, American poet and writer, pseudonym H.H.
• Helly Hansen, a Norwegian clothing maker, who uses the trademark HH
• Søren Kierkegaard, Danish philosopher and theologian, pseudonym H.H.

Science and technology

• Hh, a signalling molecule in Drosophila named for the Hedgehog signaling pathway
• hh blood group, a rare blood type
• Henderson–Hasselbalch equation, in biology and chemistry
• Herbig–Haro object, in astronomy
• Hitchhiker Program, a NASA program established in 1984
• Hodgkin–Huxley model, an electrical model of neurons

Transportation

• HH (Court Street Shuttle) a defunct line on the New York City Subway from 1936 to 1946
• HH (Rockaway Shuttle), from 1962 to 1972
• Air Hamburg, a German airline based in Hamburg (IATA code HH)

Other uses

• hh (digraph), in several languages
• His Highness, or Her Highness, a formal address
• His Holiness, or Her Holiness, a formal address
• Hamburg, Germany (ISO 3166-2 code DE-HH) and HH ("Hansestadt Hamburg"), vehicle licence plate prefix
• Hartmann House Preparatory School, an independent preparatory school in Harare, Zimbabwe
• HH, a brassiere cup size
• hh, an abbreviation for "hands high" when referring to the height of a horse
• Heirs Holdings, a Nigerian conglomerate with diversified interests
• Heywood Hill, a bookshop in London