Trichomorphology is the scientific study of the shapes of hairs. From the rigid quills of porcupines, to the soft qiviut (underhair) of muskoxen, the diversity of hair shapes among Canadian mammals illustrates how such specific epidermal structures evolved over time to fulfil various biological functions. Despite significant advancements in understanding this diversity, numerous enigmas persist, particularly regarding the evolutionary mechanisms behind hair shapes. This book, marking the inaugural Atlas dedicated to Canadian mammals, is a testament of a still budding science in the country. This chapter explores the general morphology of hairs, with a focus on the medulla, cortex, and cuticle, which are the constituents of virtually all hairs (except for a few that are devoid of medulla). To provide a general introduction to the topic, it also provides an overview of the different types of hairs and their functions. It concludes with a discussion on how trichomorphology is applied in taxonomical studies, which is the fundamental purpose of this Atlas.