The primary goal of ventilation is to provide dilution capacity for lowering the concentration of contaminant by diluting contaminants generated within a room. First is discussed the properties of the dilution curve which is the concentration as a function of the ventilation flow rate and the varying efficiency of dilution at different levels of concentration. The concept purging flow rate which is the flow rate that ventilates is introduced. In situations where the supply air is laden with the contaminant that is going to be removed it is important to distinguish between the purging flow rate and the flow rate supplied by the ventilation system. The trade-off between control of contaminant levels by ventilation and source control is addressed. Factors that may diminish the dilution capacity as short circuiting and direct loss of air are dealt with. A sensation of draught may be counteracted by the occupants’ manipulation of the ventilation system which may lead to reduced dilution capacity. Situations where there is a risk of draught occurring are identified and design parameters are introduced whose use in design can help in avoiding systems that cause draught. Communication about how ventilation works is important for avoiding mistakes and wrong expectations about what can be achieved with ventilation.