Understanding the regulation of breathing in the critical care patient is multifaceted, especially in ventilator-dependent patients who must interact with artificial respiration. Mechanical ventilation originally consisted of simple, manually-driven pump devices, but it has developed into advanced positive pressure ventilators for continuous support of patients in respiratory failure. This evolution has resulted in mechanical ventilators that deliver assist intermittently, attempting to mimic natural breathing. Recently, modes of mechanical ventilation that synchronize not only the timing, but also the level of assist to the patient's own effort, have been introduced. This article describes the concepts related to proportional assist ventilation and neurally adjusted ventilatory assist, and how they relate to conventional modes in terms of patient-ventilator synchrony.