An important point to appreciate how ventilation occurs is the concept of gas flow itself. Gas has its own characteristics, like when it is on sea level, it is different compared when it is under sea level or even above sea level. This means even the gas or air inside our lungs, e.g., oxygen and carbon dioxide, changes its characteristics on sea level, under sea level, or above sea level. In this chapter, basic mathematics and physics will be explained. They are related to gas characteristics in the lungs and also related to the tables and graphs shown on ventilator. Mechanical ventilator also shows graphs of flow, pressure, and volume. In the following chapter, all those variables which are often encountered on mechanical ventilator will be discussed; the relationship of flow and resistance to pressure, which is related to pressure from mechanical ventilator against resistance in the lungs and even the ventilator tubings, will also be in this chapter. In mechanical ventilator, there are various flow patterns, square, decelerating, and sinus waveform, which will be explained and shown further in this chapter.