Few historians other than Martin Klein, deeply familiar with the varied nature of slavery in Africa, would have been capable of compiling a historical dictionary of an institution this complex and diverse in its global manifestations. The challenge for the compiler is, inevitably, how to make sense of this diversity. In the introduction, Klein begins by comparing different types of slavery and tracing their origins in the context of political centralization. He distinguishes between two broad scenarios in which slavery historically developed. In densely populated societies like China, where there was no shortage of farm labor, slaves were retained mainly in the courts, households, and armies of the rich and powerful. But in situations where landowners faced labor shortages, slavery often became the basis for agricultural development. Examples of the latter scenario include Southeast Asia, Greece, Rome, and the slave societies of the Americas. While the introduction zooms in on the emergence and abolition of plantation slavery in the Atlantic World, the dictionary itself covers the many varieties of slavery that have existed worldwide, including several cases of military and elite slaves.

For this second edition (the first edition was published in 2002), Klein has created several new entries on modern-day slavery and also paid greater attention to slavery in Africa, Asia, and the Indian Ocean world and to Amerindian slavery, in contrast to the more familiar European and American slave systems. The dictionary is preceded by a chronology of slavery and emancipation, running from the oldest evidence of slave labor at around 3200 bce in Sumer (Mesopotamia) to the latest abolitionist movement established in Mauritania in 2008, and concludes with an updated (until 2013) fifty-page bibliography organized by theme and geographical region. The dictionary entries provide both in-text and after-text references to related topics, inviting the user to explore different histories of slavery across the globe. For instance, the section on African slavery leads to entries on honor and mining, but also to topics like the African slave trade, Central Africa, and Islam. The African slave trade connects with military slaves, which brings up the ceddo of Senegambia and tonjon of Segou and Kaarta, among others. Central Africa cross-references to São Tomé and Brazil, but also to historical figures like King Afonso I of Kongo and the Swahili merchant Tippu Tip. Islam redirects the reader to harems, eunuchs, and jihad and to places like Iran and Zanzibar. Along the way, Klein introduces several important scholars of slavery, like W. E. B. Du Bois, Eric Williams, Seymour Drescher, and Kevin Bales, founder of the anti-slavery organization Free the Slaves.

Klein's decision to expand the dictionary to cover modern forms of bondage was an obvious choice, as slavery never disappeared but rather changed in the wake of globalized efforts to ban the institution. As the work of Suzanne Miers and others has shown, the nineteenth and twentieth centuries produced a mixture of traditional and new forms of slavery, including chattel slavery, debt bondage, and contract slavery. According to Bales, modern-day slavery is very different from old-style plantation slavery, not in the least because it is based on control rather than legal ownership. This raises the question, however, whether it is possible at all to identify slavery in different historical and cultural settings by one or more constant variables.

In the introduction, Klein underlines the difficulty of creating a universal definition of slavery, given that the concept includes a wide range of relationships, which often grew out of, coexisted with, and resembled other forms of servitude, such as convict labor, pawnship, and serfdom. He proposes, nonetheless, that in general slaves were property, kinless, and acquired in an act of violence, such as warfare, kidnapping, or a legal procedure. But how do elite slaves like the Mamlukes, who ruled over Egypt for centuries, fit this definition? The families who are trapped in debt bondage in Pakistan and India today can be called slaves, but they are not kinless. By contrast, Malik Ambar lost his family when he was enslaved in Ethiopia in the sixteenth century, but could he ‘be forced to do any kind of work’ (1) once he was a military general in India? Klein more than succeeds in showing the many-sided reality of past and present slaveries worldwide. The big theoretical question to take away from the dictionary is to what extent slavery can be defined by a list of traits.