The Paraklesis is a service of supplication offered for the living, as the memorial service is offered for the departed. At its heart is a supplicatory canon, a hymn in odes, sung with psalms, hymns, and litanies. It may be addressed to any saint whose intercessions are sought, but most often it is addressed to the Most Holy Theotokos.
There are two canons to the Theotokos. The Small Paraklesis was composed by Theosterictus the Monk in the ninth century. The Great Paraklesis was composed in the thirteenth century and is attributed to a Byzantine emperor of the Laskaris line, usually Theodore II. Outside the Dormition Fast, when a Paraklesis to the Theotokos is served, it is the Small.
In the Dormition Fast, from August 1 to 14, the two are sung on alternate evenings, with some exceptions: none on Saturday evenings or on the eve of the Transfiguration, and the Great Paraklesis always on Sunday evenings. In the Russian Church the equivalent service is the moleben.

