This book is the first to provide a history of the modern Thai monarchy. The monarchy remains the single most important institution in Thai politics and arguably in Thai society as well, and it is also the most complex. Why is this the case? The last century has seen a complicated sequence of periods of waxing and waning from an absolute monarchy to a constitutional regime to a kind of hybrid system, whereby the late King Bhumibol Adulyadej came to wield political and moral authority far beyond the scope of his constitutional prerogatives. How can a quasi-absolute monarchy evolve and function effectively within a constitutional system? Moreover, the last few decades have brought a high degree of politicization of the monarchy, increasingly to its detriment, to the point that in some circles, at least, the dynasty's future was seen as increasingly uncertain even before the passing of its most revered member. In 2020, the country has witnessed public demonstrations calling for reform of the monarchy, something unprecedented in Thai history. In order to understand the present difficulties of King Vajiralongkorn, one must go back to the forced change to a constitutional monarchy in 1932, and in order to properly understand 1932, one must start as early as 1851 with his great-great-grandfather Mongkut.