Prayer is a unique and unified work in its ultimate purpose - to come closer to God to the point of permanent dialogue. It is at the same time an ascetic endeavour, but also a process, which aims at ʺinner unificationʺ, and can ultimately express a way of life - a ʺlifestyle of prayerʺ. At the end of it we can speak of an ʺinterior prayerʺ as a state. These concepts are frequently present in ascetic-mystical terminology and accompany what is called the isihastic life. In the present research we will consider the analysis and exposition of the three concepts highlighted in the title - tranquillity, interior prayer and interior unification, as they appear in the writings of St. Isaac Sirul, in the line of a succession of mystical tradition, in the context of the Solemn Year of Prayer and of the Isichasm.