This book presents the first comprehensive exploration of the state of this well studied and thus unsolved mystery of the value of the Hubble constant. Chapters covering data, systematics, independent methodologies, and theoretical explanations are authored by distinguished subject-matter experts. Tailored for scientists in the field or adjacent fields, this book provides a resource of insights and also an in-depth background sufficient for motivating future study.  Key Features:  Introduction: Written by the Nobel Laureate Adam Riess. Measurement Techniques: Gain a profound understanding of the leading Hubble constant measurement techniques, from parallax and masers to supernovae and cosmic chronometers, dissected by leading authorities on each. Systematic Errors: A deep dive into the complexities of data. Experts scrutinize potential sources of systematics that may contribute to the observed disagreements, offering a nuanced view of the challenges in Hubble constant determinations. Theoretical Explanations and Progress: A review of the theoretical explanations, some that are still standing, and some that have been ruled out; all of which have led to scientific progress toward the truth.  A Comprehensive Take: This book covers every key and influential aspect to the dilemma of the Hubble tension to date and has gathered world-leading voices on each respective topic.