Firs and pines dominated the global picture of raw materials used by the paper industry until the 1950s. At that time, the interest in introducing new species, mostly hardwoods, led researchers to intensify efforts investigating the fibrous characteristics, and their combinations, which could represent the relationship between fibres, pulp and paper. The pulp and paper industry has shown, mainly in the last two decades, a strong North-South displacement. This is to a large extent due to the favourable climate, which promotes tree development. Similarly, the paper fibres have gone from being almost exclusively softwoods from natural forests of the cold regions of the northern hemisphere, such as spruce and birch, to fast-growing species of short fibres, such as eucalyptus, and willow and poplar hybrids from plantations. These new species, which are beginning to dominate the paper panorama, not only differ from classic species in fibre length, but they present particular characteristics, such as large amounts of juvenile wood, different fibrillar angle and so on, because trees are increasingly used at a younger age. This leads us to question whether the old paradigms concerning the relationships between fibre characteristics and pulp properties are still valid or should be reviewed and updated, in which case, the basic fibre parameters, their influence in pulping and refining, and their impact on paper quality should be redefined. The purpose of this book is to survey publications of the last decade, to verify which morphological characteristics of the fibres authors currently consider relevant, in order to establish the state of the art for this topic. Relatively recent data were surveyed because of the continuous changes that occur in the species due to genetic improvement.