Mites have been traditionally treated as a Subclass (Acari or Acarina) of the Class Arachnida, but in more recent studies the taxonomic rank of mites has tended to rise.  The closest relatives of mites appear to be the Ricinuleida, and the Acari + Ricinuleida comprise the Acarinomorpha.  Shared derived characters supporting this taxon include a hexapod larval stage and a distinct capitulum or gnathosoma.  Within the Acarinomorpha are four orders, the Ricinuleida and three orders of mites: Opilioacariformes (Tetrasigmata, Notostigmata, Opilioacarida), Parasitiformes, and Acariformes.  The Opilioacariformes and Parasitiformes appear to be closely related, but convincing synapomorphies have yet to be demonstrated.  However, some authors group these two orders into a superorder (Anactinotrichida) based on the lack of actinochitin in their setae (a plesiomorphy) and oppose it to the superorder Actinotrichida (=Acariformes) which have actinochitin filled setae.

The three major lineages of mites have been recognized since the seminal studies of Grandjean in the 1930's, but well defined subgroups have emerged more slowly.  The Parasitiformes has three distinct suborders:  Ixodida (ticks), Holothyrida (holothyrans), and Mesostigmata (also Gamasina; mesostigmatans or gamasines); however, only the last is further divided into what are traditionally called cohorts (infrasuborders).  Eleven of these parasitiform cohorts occur in soil and are differentiated in this key.

The Acariformes contains about 4/5ths of the 50,000+ described species of mites, and a morass of subdivisions.  The currently recognized suborders include Oribatida (Cryptostigmata, Oribatei), Astigmata (Acaridida), Endeostigmata, and Prostigmata (Actinedida).  Unfortunately, the Astigmata and most of the Endeostigmata should be grouped with the Oribatida (a group called the Sarcoptiformes) and two families of the Endeostigmata (comprising the Sphaerolichida) with the Prostigmata (producing the Trombidiformes).  In this key, the Sphaerolichida and 6 cohorts of Prostigmata senso strictu are distinguished, as well as the Astigmata, the unusual dispersal morphs of the Astigmata (Astigmatan hypopi), Endeostigmata, and 6 cohorts of Oribatida.