The earliest evidence of human occupation in Hertfordshire come from a gravel pit in Rickmansworth. The finds (of flint tools) date back 350,000 years, long before Britain became an island. People have probably lived in the land now called Hertfordshire for about 12,000 years, since the Mesolithic period.
In the Iron age, a Celtic tribe called the Catuvellauni occupied Hertfordshire. Their main settlement (or oppidum) was Verlamion on the River Ver (near present-day St Albans).
There is a wealth of Iron Age burial sites in Hertfordshire, making it a place of international importance in Iron Age study.The large number of sites of all types indicates dense and complex settlement patterns immediately prior to the Roman invasion.
In 55 BCE when the Romans first attempted to invade Britain, the Catuvellauni (which is Brythonic for "Expert Warrior") were the largest British tribe. Caesar's report to the Senate said that "Cassivellaun" (Cassivellaunus) was leader of the Britons, and Cassivellaunus' headquarters were near Wheathampstead in Hertfordshire. On Caesar's second invasion attempt in 54 BCE, Cassivellaunus led the British defensive forces. The Romans besieged him at Wheathampstead, and partly because of the defection of the Trinovantes (whose King Cassivellaunus had had murdered), the Catuvellauni were forced to surrender.
A number of Roman Roads run through Hertfordshire including Watling Street and Ermine Street. The ancient trackway, the Icknield Way also runs through Hertfordshire. These are three of the "four highways" of medieval England (the other being the Fosse Way, which does not run through Hertfordshire) which were still the main routes through the country more than a thousand years later.