Thanks to their long necks and tails, diplodocids were among the longest sauropods, with some species such as Supersaurus vivianae and Diplodocus hallorum estimated to have reached lengths of 30 meters (100 ft) or more.

The heads of diplodocids have been widely depicted with the nostrils on top due to the position of the nasal openings at the apex of the skull.

Studies by Lawrence Witmer (2001) indicated that, while the nasal openings were high on the head, the actual, fleshy nostrils were situated much lower down on the snout.

[7] Diplodocids had long necks, which could reach an estimated length of 15 metres (50 ft) in the largest, longest-necked species.

They probably used their teeth to crop off food, without chewing, and relied on gastroliths (gizzard stones) to break down tough plant fibers (similar to modern birds).

Unilateral branch stripping is the most likely feeding behavior of Diplodocus,[19][20][21] as it explains the unusual wear patterns of the teeth (coming from tooth–food contact).

With the elongated preorbital (in front of the eyes) region of the skull, longer portions of stems could be stripped in a single action.

[18] Also, the palinal (backwards) motion of the lower jaws could have contributed two significant roles to feeding behaviour: 1) an increased gape, and 2) allowed fine adjustments of the relative positions of the tooth rows, creating a smooth stripping action.

It was concluded that the proposal that its dentition was used for bark-stripping was not supported by the data, which showed that under that scenario, the skull and teeth would undergo extreme stresses.

A study by Griebeler et al. (2013) examined long bone histological data and concluded that the diplodocid MfN.R.2625 weighed 4,753 kilograms (5.2 short tons), reached sexual maturity at 23 years and died at age 24.

The same growth model indicated that the diplodocid MfN.R.NW4 weighed 18,463 kilograms (20.4 short tons), and died at age 23, before reaching sexual maturity.

[24] An unnamed diplodocid specimen from the Morrison Formation nicknamed "Dolly" shows evidence of a throat infection that created cauliflowered bone in the vertebral air sacs.

There is some circumstantial evidence supporting this as well: a number of diplodocids have been found with fused or damaged tail vertebrae, which may be a symptom of cracking their tails: these are particularly common between the 18th and the 25th caudal vertebra, a region the authors consider a transitional zone between the stiff muscular base and the flexible whiplike section.

[31][34] Some dinosaurs have been considered diplodocids in the past but have not been found to be members of that group in later, larger analyses of the family's relationships.

[32][35] Amphicoelias was traditionally considered a diplodocid due to its similar anatomy, but phylogenetic studies showed it to be a more basal member of the Diplodocoidea.

However, a subsequent analysis by Whitlock in 2011 showed that Supersaurus is slightly closer to Diplodocus than to Apatosaurus, and that Suuwassea is actually a primitive dicraeosaurid.