The evidence in support of intraocular pressure (IOP) lowering to reduce risk of glaucoma onset or progression is strong, although the amount and quality of IOP reduction is less well defined. The concept of a target IOP includes a percentage reduction, calculated IOP, or a predetermined IOP figure or range. Yet none of these strategies have been validated. In addition, our understanding of the way IOP influences glaucoma risk is continuously evolving. Examples of this include the data on IOP fluctuation and lamina cribrosa and cerebrospinal fluid pressure differentials. That these variables are not included in target IOP calculation potentially undermines its accuracy and usefulness. We summarize the evidence for target IOP, new developments in our understanding of IOP and glaucoma pathogenesis, as well as emerging strategies for setting targets and assessing response to treatment.