The troodontids are best characterized as small, rather delicate theropods with relatively the largest brains of any classic dinosaur, long and slender legs, and unusual teeth that have been mistaken for those of the ornithischian group Pachycephalosauria.  These teeth have led some researchers to suggest that they were omnivorous.  Like ornithomimosauroids, they had large eyes.  Troodontidae has proven hard to classify, and can be linked by various characteristics with Spinosauridae, Dromaeosauridae, Oviraptorosauria, and Ornithomimosauria.  In the recent past, they were often combined with the ornithomimosaurians into a taxon called Bullatosauria, meaning "inflated reptiles" (referring to a "swollen" bone in the braincase).  Part of the difficulty is that they are still rather poorly known; only one really complete skeleton, that attributed to Sinornithoides, has been uncovered.  In addition, until recently there had been no good basal troodontid remains known, hindering comparisons.  With the discovery of Sinovenator, however, Troodontidae seems to mesh with Dromaeosauridae in Deinonychosauria, just as in the early 1980s. 
    For a long time, this family was known as Saurornithoididae, because Troodon was thought to be a dubious pachycephalosaurid.  It may return to this name if recent investigations into Troodon show that the type teeth are not "troodontid".