A worldwide shortage of calf rennet for cheese production has existed for several decades. Bovine pepsin and to a certain extent porcine pepsin and plant coagulants have been used as rennet substitutes, but these have not been commercially successful owing to their extensive proteolytic nature and other inherent drawbacks. Investigations to develop other alternatives have resulted in the introduction of various microbial coagulants which have found markets in several countries despite certain shortcomings when compared with traditional calf rennet. However, in some countries, microbial coagulants have not been accepted for regular cheese manufacture because they are believed to result in a reduced yield and a lower quality product; this is particularly applicable to cheddar cheese manufacture. Immobilization of proteases for milk coagulation has received renewed thrust; this may convert renneting of milk to a continuous operation. However, this technique has yet to be applied on an industrial scale; such a process is not likely to be successful since immobilized enzyme technology is dependent on the ability of a small, rapidly diffusing substrate to move quickly around the immobilized enzyme. In contrast, casein micelles are large and diffusion is slow, so the proteolysis rate is very slow. Microbial rennets offer an attractive target for genetic engineers since it may be possible to alter their structure/function characteristics to match those of calf rennet; application of site‐directed mutagenesis could be particularly rewarding. The term “calf rennet,” in the cheese industry, generally refers to an enzyme extract obtained from the fourth stomach (abo‐masum) of 10‐ to 30‐day‐old calves and used to coagulate milk for cheese production. The purified milk‐clotting enzyme present in crude rennet preparations is known as “rennin” or “chymosin.” The designation “chymosin” is now recommended in the international enzyme nomenclature (renin is associated with hypertension and is derived from the kidney). Calf rennet is referred to as “animal rennet” frequently, the term “chymase” appears in the older literature as a synonym for rennet. In more general usage, however, any milk‐clotting enzyme preparation yielding a relatively stable curd is designated as rennet. Christian Hansen is credited with the first industrial production of rennet in 1874 (1). This enzyme was called chymosin (EC 3.4.23.4); it belongs to the group of aspartic proteases and is the standard against which all other types of milk‐clotting enzymes are compared. Since the origin of cheese production, the manufacturing process has always needed soluble enzymes to clot the milk; it has been adapted to the properties of calf rennet, an essential enzyme for this purpose. Rennet coagulates milk rapidly at its natural pH with little further degradation of the milk proteins. The zymogen, prorennin, is converted to chymosin at pH below 5.0, but optimally at pH 2.0 (2). Chymosin, a strong protease, has been crystallized (3) and detailed crystallographic studies of the enzyme have been reported by Bunn et al. (4). Besides a slightly high proteolytic activity, another disadvantage of this enzyme is that it is extracted from the abomasum of the unweaned calf. As the calf ages, chymosin is replaced by pepsin, although in cattle, the secretion of chymosin never comes to a complete stop. Although pepsin can clot milk, it has a tendency to result in higher fat losses because the curd formed has a more open, looser structure than that formed with chymosin; the cheese produced also has a softer body than desired. Pepsin and a commercial product under the trade name Metroclot (5, 6) have, however, been used for the production of a variety of cheeses in the past. The kinetic properties and amino acid composition of chymosin and pepsin have been extensively studied. There is more or less a consensus of opinion in favor of chymosin as the enzyme for cheese making; presently three companies are producing calf chymosin through recombinant DNA technology. The cloned chymosin preparations produced by different microorganisms have been tested in various countries for cheese manufacture. No major differences could be detected among cheeses made with cloned chymosin and those made with the natural enzyme. This review describes the past, present, and future status of rennet substitutes utilized as milk coagulants and applications of modern biological tools to strain improvement and process development.