Advances in Hair Styling 416 consumer, such as (but not limited to) cutting the hair into a particular shape, using the correct styling product or regimen of products, and proper use of appliances to hold the hair in place. The purpose of this chapter is to explore hair styling from a technical point of view without losing sight of practicality. To this end, many factors important in hair styling will be discussed, such as variation in hair morphology due to hair type, the effect of the mechanical properties of hair, the physicochemical features of hair products and functional ingredients that are applied to hair, the physical and mechanical effects of styling appliances, aspects of polymer-hair fiber assembly interactions, and the important testing techniques use to judge efficacy. This then forms a very intricate array of factors that make the subject of hair styling complex. Because of this, studying the subject has to be approached as a multivariable system with many interacting components that are very much dependant on each other. In this chapter many of these factors and their importance to the hair styling process will be explained, but more importantly the advances in the subject of hair styling will come primarily from getting a good grasp of this complexity and how this is going to take us into the next trend for the future. Once this is achieved, the cosmetic chemist can design the best ingredient or formula or test method to progress further toward his or her objectives. Influence of Hair Morphology on Hair Styling Although this chapter does not go into detail on hair morphology, it must be stressed here that morphology is a key parameter that influences hair styling, e.g. it is easier to style shorter hair than longer hair. Similarly, hair with higher diameter or greater thickness is a lot more difficult to put into a certain shape/style. It is also harder to hold it in that shape, which is defined as “style retention.” Hair density is also inversely proportional to the ease of hair styling. The cuticle edge interval is another factor. Hair that has a less frequent cuticle edge interval is more flexible. Flexible hair is easy to bend and relatively easy to manipulate in certain shapes/styles. This feature has significant