The ability to practice diplomacy is one of the defining elements of a state, and diplomacy has been practiced since the first city-states were formed millennia ago. For most of human history diplomats were sent only for specific negotiations, and would return immediately after their mission concluded. Diplomats were usually relatives of the ruling family or of very high rank in order to give them legitimacy when they sought to negotiate with the other state.

One notable exception involved the relationship between the Pope and the Byzantine Emperor; papal agents, called apocrisiarii, were permanently resident in Constantinople. After the 8th century, however, conflicts between the Pope and Emperor (such as the Iconoclastic controversy) led to the breaking of close ties.

Modern diplomacy's origins are often traced to the states of Northern Italy in the early Renaissance, with the first embassies being established in the thirteenth century. Milan played a leading role, especially under Francesco Sforza who established permanent embassies to the other city states of Northern Italy. It was in Italy that many of the traditions of modern diplomacy began, such as the presentation of an ambassadors credentials to the head of state.

From Italy the practice was spread to the other European powers. Milan was the first to send a representative to the court of France in 1455. However, Milan refused to host French representatives fearing espionage and that the French representatives would intervene in its internal affairs. As foreign powers such as France and Spain became increasingly involved in Italian politics the need to accept emissaries was recognized. Soon the major European powers were exchanging representatives. Spain was the first to send a permanent representative; it appointed an ambassador to the Court of England in 1487. By the late 16th century, permanent missions became customary. The Holy Roman Emperor, however, did not regularly send permanent legates, as they could not represent the interests of all the German princes (who were in theory subordinate to the Emperor, but in practice independent).

During that period the rules of modern diplomacy were further developed. The top rank of representatives was an ambassador. At that time an ambassador was a nobleman, the rank of the noble assigned varying with the prestige of the country he was delegated to. Strict standards developed for ambassadors, requiring they have large residences, host lavish parties, and play an important role in the court life of their host nation. In Rome, the most prized posting for a Catholic ambassador, the French and Spanish representatives would have a retinue of up to a hundred. Even in smaller posts, ambassadors were very expensive. Smaller states would send and receive envoys, who were a rung below ambassador. Somewhere between the two was the position of minister plenipotentiary.

Diplomacy was a complex affair, even more so than now. The ambassadors from each state were ranked by complex levels of precedence that were much disputed. States were normally ranked by the title of the sovereign; for Catholic nations the emissary from the Vatican was paramount, then those from the kingdoms, then those from duchies and principalities. Representatives from republics were considered the lowest of the low. Determining precedence between two kingdoms depended on a number of factors that often fluctuated, leading to near constant squabbling.

Ambassadors, nobles with little foreign experience and no expectation of a career in diplomacy, needed to be supported by large embassy staff. These professionals would be sent on longer assignments and would be far more knowledgeable than the higher-ranking officials about the host country. Embassy staff would include a wide range of employees, including some dedicated to espionage. The need for skilled individuals to staff embassies was met by the graduates of universities, and this led to a great increase in the study of international law, modern languages, and history at universities throughout Europe.

At the same time, permanent foreign ministries began to be established in almost all European states to coordinate embassies and their staffs. These ministries were still far from their modern form, and many of them had extraneous internal responsibilities. Britain had two departments with frequently overlapping powers until 1782. They were also far smaller; France, which boasted the largest foreign affairs department, had only some 70 full-time employees in the 1780s.

The elements of modern diplomacy slowly spread to Eastern Europe and Russia, arriving by the early eighteenth century. The entire edifice would be greatly disrupted by the French Revolution and the subsequent years of warfare. The revolution would see commoners take over the diplomacy of the French state, and of those conquered by revolutionary armies. Ranks of precedence were abolished. Napoleon also refused to acknowledge diplomatic immunity, imprisoning several British diplomats accused of scheming against France.

After the fall of Napoleon, the Congress of Vienna of 1815 established an international system of diplomatic rank. Disputes on precedence among nations (and therefore the appropriate diplomatic ranks used) persisted for over a century until after World War II, when the rank of ambassador became the norm. In between that time, figures such as the German Chancellor Otto von Bismark were renowned for international diplomacy.

Diplomacy is closely linked to espionage or gathering of intelligence. Embassies are bases for both diplomats and spies, and some diplomats are essentially openly-acknowledged spies. For instance, the job of military attachés includes learning as much as possible about the military of the nation to which they are assigned. They do not try to hide this role and, as such, are only invited to events allowed by their hosts, such as military parades or air shows. There are also deep-cover spies operating in many embassies. These individuals are given fake positions at the embassy, but their main task is to illegally gather intelligence, usually by coordinating spy rings of locals or other spies. For the most part, spies operating out of embassies gather little intelligence themselves and their identities tend to be known by the opposition. If discovered, these diplomats can be expelled from an embassy, but for the most part counter-intelligence agencies prefer to keep these agents in situ and under close monitoring.

The information gathered by spies plays an increasingly important role in diplomacy. Arms-control treaties would be impossible without the power of reconnaissance satellites and agents to monitor compliance. Information gleaned from espionage is useful in almost all forms of diplomacy, everything from trade agreements to border disputes.