People tend to enjoy listening to music or watching television, sleeping at night and celebrating birthdays. Plants tend to grow and thrive in sunlight and mild temperatures. We also know that tendencies are not perfectly regular and that there are patterns in the natural world, which are reliable to a degree, but not absolute. What should we make of a world where things tend to be one way but could be another? Is there a position between necessity and possibility? If there is, what are the implications for science, knowledge and ethics? This book explores these questions and is the first full-length treatment of the philosophy of tendencies. Anjum and Mumford argue that although the philosophical language of tendencies has been around since Aristotle, there has not been any serious commitment to the irreducible modality that they involve. They also argue that the acceptance of an irreducible and sui generis tendential modality ought to be the fundamental commitment of any genuine realism about dispositions or powers. It is the dispositional modality that makes dispositions authentically disposition-like. Armed with this theory the authors apply it to a variety of key philosophical topics such as chance, causation, epistemology and free will.