Caesar Rules, by Olivier Hekster, sets itself the very ambitious goal of defining the indefinable, filling the gap left by the lack of definition of what the Roman Emperor was and how he was perceived. In this book Hekster is remarkably successful in colouring in this blank image by not just relying on historical source material, but by going further into the image of the emperor left for posterity in art and in numismatics. His engaging analysis of this world-ruling enigma will certainly be the starting point for many years of continued research into the nature of what it was to be a Roman Emperor.

Hekster's work is singular in that it is a new application of study as it departs from the previous studies on what it was to be Emperor. Most notably it is a departure from the seminal work of Fergus Millar published in 1977. Hekster's change of emphasis is to focus on the changing nature of the concept of Emperor from a time when no one thought of such a thing even existing to going beyond the static nature of the period of Constantine to the eventual reign of Justinian where the Western Empire was fading from the imperial view. Whether you called him ‘Caesar’, ‘Augustus’, ‘Imperator’ or ‘Basileus’, Hekster noted that the role of the man went far beyond the civil or even the legal – past an empirical rational definition to a quasi-divine figure that was just as much religious as he was soldier in nature.

In Caesar Rules we find that the Roman Emperor was defined by not only what he did but also by those people that he had around him at the Imperial Court. From senators and bishops to wives and mothers, all helped or hindered the changing nature and appearance of the man at the apex of the Roman world. In Chapter 1 Hekster explores how the emperor was portrayed in public. This is done by examining how the emperor was portrayed in art, sculpture and on coinage, from the titles he used, or the offices he held to his interactions with the people both in small intimate groups or large public spectacles; a redefined image of the Roman Emperor emerges. This changing image (which is reflected by the latest trends in archaeology) is defined throughout the first chapter and then condensed and expanded in later chapters to follow. The first chapter is particularly successful in challenging notions that had been widely held such as the distinction between ‘official’ and ‘unofficial’ names. Indeed, throughout the whole work the picture that emerges is one where such representations are actually the result of negotiations not just of an egomaniacal ruler imposing his will. The ‘Changing Caesar’ is indeed a far more subtle creature.

Chapter 2 continues by showing that of all the things in Caesar Rules the term and title of Augustus is the most constant throughout this 600-year examination of the man and the office. The focus continues on more historically known roles such as the civic, military and religious functions that the emperor traditionally embodied. These are more commonly brought down to us through the most widely read historians active in the Roman Empire. Hekster does this very well given the complex changes in the function of the emperor in the religious sphere after the advent of Christianity in the reign of Constantine. He condenses this subject without losing any of the important contributions this makes to the changing image of the emperor. This is indeed masterly as the voluminous nature of the time period from 312 AD could well have consumed the whole chapter and forced other aspects of rule to be highly neglected.

Chapters 3 and 4 deal with relationships with the emperor and are thus the most different from the traditional scholarly approaches of the study of the Imperium. It focuses on the relationships of the Imperial Court and family as well as exterior special relationships with the emperor. It is thus a unique blend of intrigue, discord, harmony and propaganda. In his treatment of these subjects Hekster shows us again the workings of a master scholar. He moves effortlessly from Julio-Claudian family dynamics to the actions of Christian Empresses in the 5^th and 6^th centuries AD. This description shows us how fluid the continuity of Roman Imperial history was. Again, Hekster uses a very effective blend of both historical and visual sources to prove his argument. These are indeed a treat to any reader. With regard to the spatial relationships, Hekster pays equal amounts of attention to both Rome and Constantinople, showing the reader the substantive challenges that were faced given each city's unique geography to the possibilities of how the emperor could interact with his subjects. Even here, re-evaluation of traditional views can be seen: these once seemingly static scholarly pictures are infused with new life and new possibilities of continuing research. Just one example could be the significant periods of time that the Roman Empire had two or more co-emperors or Caesars.

Caesar Rules is meticulously researched. It presents the reader with excellent images, datasets and family trees. These have all been expertly chosen by Hekster, and this does more than just an admirable job of creating this book. It goes far beyond. The book manages to incorporate the slightest nuance with a sort of warm embrace that will evoke new scholarship and further the debate on the idea and notion of who or what the Roman Emperor was.