Chapter 8 253 more comparable to real life situations. This is a well-established discipline, and the work shown here describes applications of these principles to the study of hair breakage. In doing so, it will be shown how it becomes possible to model fiber breakage, and to compare and contrast the influence and importance of various contributors. Ultimately it becomes possible to predict the likelihood for failure under different conditions and after various treatments. The majority of this chapter will discuss results from single fiber fatigue experiments which provide fundamental insights into the contributing factors for hair breakage. These experiments often proceed very slowly, and patience is required in acquiring the essential data. As such, results and findings described herein represent ongoing work which, hopefully, illustrates the nature of insights that are produced however, absolute numerical values may have changed somewhat from those reported previously2 (and will go on being revised) as data sets continue to increase in size. Similarly, theories may also need refining as related experiments are added. Part 1: Single Fiber Fatigue Testing Experimental: The use of a homemade single fiber fatiguing device for testing hair was described in the scientific literature in 1984.3 However, commercially available equipment now exists (Dia- stron CYC800, Dia-stron Limited, Andover, UK) and was used in performing the work described herein. Comprehensive descriptions of the device have been reported,4,5 as has a more detailed overview of experimental issues and considerations.2 As such, this chapter provides a brief overview of these areas, but focuses primarily on novel findings that arise from these experiments. Figure 1 shows the testing portion of the Diastron CYC800 instrument. Individual hair fibers are crimped between brass ferrules and mounted in the sample tray. From here, the instrument automatically picks up the fibers and transports them to the testing station. The instrument then applies a user defined repeating force to each fiber until breakage occurs. The broken fiber is returned to the sample tray, followed by loading of the next sample.