When the membrane in a flow battery or fuel cell is removed, the result is a fluid–fluid interface across which selective ion exchange must occur with minimal reactant crossover. Here, we review five major approaches to the design of membraneless cells reported in the recent literature (from 2004 to mid-2016), including our own contributions. Although most of the reviewed designs were originally developed for microfluidic fuel cells, we proceed to discuss the potential applicability of these designs as membraneless redox flow batteries, given their similarity to fuel cells. The various designs exhibit noticeable performance differences due to differences in their redox couple chemistries, electrolyte compositions, and flow architectures. Therefore, we discuss eight performance metrics that can be determined from cell operation data and electrolyte cost indices and can be used for systematic comparison of various designs. Using these metrics, we present an outlook for promising designs in terms of their potential for wide acceptance or commercialization, with the aim of directing future research.