This review represents an attempt to summarize and discuss some of the literature on the batch operated internal rubber mixer, with the primary view of studying processing conditions and how they influence efficiency and productivity of the process. The scope does not include materials to be mixed, their physical state, order of loading the mixer, compounding principles, systems layout, machine design, automation, mechanization, labor utilization, or other aspects of the mixing process. The discussion is limited to mixing and processing conditions connected with typical compounds, normally based on general-purpose rubbers, which today comprise by far the largest share of rubber batches mixed. It is assumed that the reader is acquainted with the mixing process and with rubber technology in general. The principal elements of the internal rubber mixer are sketched in Figure 1. Some scattered and not very coherent data on important makes of rubber mixers are collected in Tables II, III and IV and referred to in the following discussions. Since the different machines may have different applications, caution should be used in making intercomparisons. It is hoped that this review will contribute to the understanding of the mixing process, and its efficient utilization. A list of problem areas is presented at the end with the hope of stimulating future research.