For over three decades, opposed-jets reactors have been applied particularly for fast complex chemical reactions where mixing affects the final product distribution. The first opposed-jets reactors, i.e., confined impinging jets (CIJ), had cylindrical geometries. With the advent of microreactors, a new type of prismatic opposed-jets reactors has been introduced, the so-called T-jets reactors. The state of the art of the know-how on mixing in CIJ reactors as well as the current state of the art of T-jets reactors is described. T-jets and CIJ are generally small-scale devices, ensuring that these units can mix fluids within very short mixing times on the order of milliseconds. The small scale has the disadvantage of compromising the reactors' throughput. This fact can prevent the dissemination of this type of mixers to applications where conventional reactors, such as stirred tanks, are used. Attention is paid to the issue on how to scale up opposed-jets reactors without compromising mixing times. A short analysis of the main research questions to be answered in this field is made.