After a brief introduction, this contribution will discuss practicalstrategies to exploit the unique science potential of closure phasesin optical interferometry, especially in the case of only threetelescopes when direct imaging is near-impossible. Prototype sourcemorphologies will be investigated in detail (binary and "dust shell +star"), and will illustrate how closure phase measurements can be usedboth for "Precision Interferometry" as well as qualitatively newprobes of circumstellar environments. During this meeting, the notionof a "Closure Differential Phase" came to light, and someinteresting properties of this quantity are here introduced.