Asaphida is a large, morphologically diverse order of trilobites that are found in strata dated from the Middle Cambrian boundary to the Upper Ordovician.

The Asaphida generally have cephalon (head) and pygidium (tail) parts similar in size, and most species have a prominent median ventral suture. Heads are often flat, and carapace furrows in the head area are often faint or not visible. Thoracic segments typically number 5 - 12, though some species have as few as 2 and some as many as 30. They also generally have a wide doublature, or rim, that surrounds the cephalon. Eyes are typically large.

The Order Asaphida has six superfamilies: Anomocaroidea, Asaphoidea, Dikelokephaloidea, Remopleuridoidea, Cyclopygoidea, Trinucleioidea.

One Asaphida line, the superfamily Asaphoidea, shows a continuous evolution of eyestalks, from ones with mere eyestubs to Asaphus kowalewski, a trilobite popular with collectors that sported long, eye-tipped stalks. This line is found in the Middle Ordovician Asery Level deposits of the Wolchow River region near Saint Petersburg, Russia. During the Ordovician, what is now Eastern Europe was a shallow inland sea. This eye-stalk development is believed to be an adaptation to changes in turbidity during this time, with eye-stalked trilobites like Asaphus kowalewski presumably arising in a time of increased turbidity. One thought is that this trilobite may have lain in wait for prey buried in the bottom sediment with only its 14 mm periscope eyestalks protruding. The last descendents of this line of Asaphus disappear in the layers of the Upper Ordovician.