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J. Cosmet. Sci., 72, 99–133 (January/February 2021) 99 A Review of Permanent Waving and Perm Chemistry TREFOR EVANS , TRI-Princeton, Princeton, NJ Accepted for publication August 31, 2020 . Synopsis Although traditional perm chemistry has been around for over 70 years, it still arguably remains our best means of permanently altering the shape of hair. The hullabaloo created by the so-called Brazilian keratin treatments has led to resurgence in research activities in this area—and so an overview of the incumbent technology would seem pertinent. This review highlights underlying chemistry responsible for the success of traditional thioglycolate solutions but makes the case that our knowledge gap lies with the kinetics of the process. To this end, the merits and limitations of the single-fi ber tensile kinetic approach are addressed— where, despite seemingly questionable assumptions, logical outcomes are obtained from systematic experiments intended to probe the validity of this method. Furthermore, results demonstrate how considerably different rates can arise between a common perm solution and hair from different sources. It is therefore proposed that “resistant” hair is a consequence of slow reaction rates which lead to an insuffi cient number of cystine disulfi de bonds being broken during the processing time. Additional analysis of these kinetic data suggests the presence of different mathematical models that describe the progression of the process with varying experimental conditions. The reproducibility of results and the frequency with which models arise add confi dence to fi ndings. INTRODUCTION To product developers, hair stylists, and consumers alike, the word perm conjures up images of noxious chemical treatments that impart a degree of wave or curl to otherwise straight hair. The chemistry behind this transformation is the primary focus of this review article, yet it is worth remembering that the word itself is a truncation of the term permanent wave, and whereas thiol chemistry is the most common means of achieving this end in theory, it could apply to any process that promotes an enduring makeover. By means of illustration, the origin of the expression is often traced back to the late 19th century and the use of extreme heat to change the shape of hair. Marcel Grateau is gener- ally credited with the introduction of hot curling tongs in 1872 that produced what be- came known as a Marcel wave. The introduction of chemical reagents is usually attributed to the work of Charles Nessler who in 1905 used a combination of borax (sodium borate) and heat to produce curls in hair. However, this procedure took many hours to perform and necessitated the use of hot, heavy metal curling rods. Accordingly, complex chandelier- like devices were needed to deliver the heat, support the weight, and keep the burning Address all correspondence to tevans@triprinceton.org.