Macro-mixing is an important unit operation in many process industries

and traditionally it is characterized with sort of mixing time. After academic and applied research of about 50 years

huge amount of data and knowledge has been accumulated from theoretical and experimental studies by chemical engineering community. Nevertheless

a strong sense of subjectivity is embedded in the definition of macro-mixing time and the relevant methods of experimental measurement

and the approach based on point-wise injection of tracer and point-wise detection fails to quantify the global behavior of mixing in the whole reactor. In this review

a few important topics related to the nature

definition and measurement of macro-mixing are scrutinized

the hidden defects are unveiled

and some worthwhile topics and useful suggestions are proposed to promote better thorough research on macro-mixing in single-phase and multiphase chemical reactors.