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Heptageniidae (Synonym: Ecdyonuridae) is a family of with around 500 described species with a mainly Holarctic distribution. These are generally rather small mayflies with two long tails. The wings are usually clear with prominent venation although species with variegated wings are known. As in most mayflies, the males have large compound eyes but not divided into upper and lower parts (the group is sometimes referred to as flat-headed mayflies).

Heptageniids breed mainly in fast-flowing streams (an alternative common name is stream mayflies) but some species will use still waters. The nymphs have a flattened shape and are usually dark in color. They use a wide range of food sources with herbivorous, scavenging and predatory species known.

The Family Heptageniidae is a member of the Superfamily Baetoidea. Here is the complete "parentage" of Heptageniidae:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Animalia Linnaeus, 1758 - animals
    • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
      • Branch: Protostomia Grobben, 1908 - protostomes
        • Infrakingdom: Ecdysozoa Aguinaldo Et Al., 1997 Ex Cavalier-Smith, 1998 - ecdysozoans
          • Superphylum: Panarthropoda Cuvier
            • Phylum: Arthropoda Latreille, 1829 - Arthropods
              • Subphylum: Mandibulata Snodgrass, 1938
                • Infraphylum: Atelocerata Heymons, 1901
                  • Superclass: Panhexapoda
                    • Class: Insecta C. Linnaeus, 1758 - Insects
                      • Subclass: Dicondylia
                        • Infraclass: Pterygota - Winged Insects
                          • Order: Ephemeroptera Hyatt & Arms, 1891 - Mayflies
                            • Suborder: Schistonota
                              • Superfamily: Baetoidea
                                • Family: Heptageniidae Needham, 1901 - Stream Mayflies