For modern diplomatic consuls, see diplomatic consulate.
Under the Republic the minimum age of election to consul for patricians was 40 years of age, for plebeians 42. Two consuls were elected each year, they served together with veto power over each other's actions, and the year of their service was known by their names. For instance, the year we commonly call 59 BC was called by the Romans "the consulship of Caesar and Bibulus," since the two colleagues in the consulship were Julius Caesar and Marcus Calpurnius Bibulus (although Caesar dominated the consulship so thoroughly that year that it was jokingly referred to as "the consulship of Julius and Caesar").
In Latin, "consules" means "those who walk together". If a consul died during his term (not uncommon when consuls were in the forefront of battle), another would be elected, and be known as a suffect consul (cos. suff.).
The office of consul was believed to date back to the traditional establishment of the Republic in 509 BC, although the early history is partly legendary, and the succession of consuls is not continuous in the 5th century. Consuls executed both religious and military duties; the reading of the auguries was an essential step before leading armies into the field.
During times of war, the primary criterion for consul was military skill and reputation, but all times the selection was politically charged. Initially only patricians could be consuls, and later the plebeians won the right to elect one of their own; the first plebeian consul was Lucius Sextius, in 366 BC.
With the passage of time, the consulship became the normal endpoint of the cursus honorum, the sequence of offices pursued by the ambitious Roman.
When Augustus established the Empire, he changed the nature of the office, stripping it of most if not all of its powers. While still a great honor, and a requirement for other offices, many consuls during his long rule would resign part way through the year, to allow other men to hold the fasces as suffects. Those who held the office on January 1, known as the consules ordinarii had the honor of associating their names with that year. As a result, about half of the men who held the rank of praetor could also reach the consulship. Sometimes these suffect consuls would in turn resign, and another suffect would be appointed. This reached its extreme under Commodus, when in 190 twenty-five men held the consulship.
Another change under the Empire was that Emperors frequently appointed themselves, proteges, or relatives without regard to the age requirements. For example, Honorius was conferred the consulship upon his birth.
Holding the consulate was apparently such an honor that the break-away Gallic Empire had its own pairs of consuls during its existence (260 - 274). The list of consuls for this state is incomplete, drawn from inscriptions and coins.
One of the reforms Constantine I made was to assign one of the consuls to the city of Rome, and the other to Constantinople. Therefore, when the Roman Empire was divided into two halves on the death of Theodosius I, the emperor of each half acquired the right of appointing one of the consuls -- although one emperor did allow his colleague to appoint both consuls for various reasons. As a result, after the formal end of the Roman Empire in the West, many years would only be named for a single consul. This rank was finally allowed to lapse in the reign of Justinian: first with the consul of Rome in 534, Decimus Theodorius Paulinus, then the consul of Constantinople in 541, Flavius Basilius Junior.
For a complete list of Roman consuls, see: