An overview of the mixing performances of micro T-mixers operating with a single fluid is presented. The focus is on the relationship between flow features and mixing. Indeed, T-mixers are characterized by a variety of regimes for increasing Reynolds numbers; they are briefly described, in particular in terms of the three-dimensional vorticity field, which can explain the different mixing performances. The effects of changes in the aspect ratios of the channels are also reviewed. The role of instability and sensitivity analyses in highlighting the mechanisms of onset of the different regimes is then described. These analyses also suggest possible geometrical modifications to promote mixing. We focus on that consisting in the downward tilting of the inlet channels (arrow-mixers). Arrow-mixers are interesting because the onset of the engulfment regime is anticipated at lower Reynolds numbers. Hence, the mixing performances of arrow-mixers with varying Reynolds number are described.