In-tube boiling and condensing flows, a subset of the area of phase-change heat transfer, are reviewed here with emphasis on their annular regime realizations in innovative devices. The review is also relevant because of recently reported experimental and technological approaches that make it feasible for annular/stratified regimes to cover the entire lengths of millimeter scale flow-boilers and flow-condensers. The review article’s content also relates to new ways of addressing challenges that have emerged in the area of high heat-flux (500-1000 W/cm2 or greater) cooling of data centers, super computers, laser weapons, and other devices. The article summarizes current knowledge base and provides a design example each for steady annular operations of flow-boilers and flow-condensers. Furthermore, the article deals with some recent breakthroughs in the simulation of annular boiling/condensing flows and how such approaches can be extended – in conjunction with well-planned and parallel experiments – to further develop the science and innovative applications associated with steady or pulsatile operations of annular flow-boilers and flow-condensers.