The paper is concerned with identification of the key mechanisms controlling deflagration-to-detonation transition in confined and unconfined gaseous systems. The issue of thermal runaway triggered by positive feedback between the advancing flame and the flame-driven precursor shock is revisited. A new mechanism for parametric transition based on the flame-speed sensitivity to pressure changes is discussed. Depending on the parameters of the system the transition may occur either within the subsonic, sonic or supersonic range of deflagrations. In the latter case the deflagration obeys the classical Chapman-Jouguet (CJ) condition. In a certain parameter range the adopted model reproduces the experimentally known situation where the transition occurs close to the sonic point. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.