Hertfordshire is one of the home counties and is bordered by Bedfordshire to the north, Cambridgeshire to the north-east, Essex to the east, Buckinghamshire to the west and Greater London to the south. The county town is Hertford.

Hertfordshire is located immediately to the north of Greater London and is part of the East of England Government Office Region. Much of the county is part of the London commuter belt. To the east of Hertfordshire is Essex, to the west is Buckinghamshire and to the north are Bedfordshire and Cambridgeshire.

The county's boundaries were fixed by the Counties (Detached Parts) Act 1844 which eliminated exclaves. They were amended when, in 1965 under the London Government Act 1963, East Barnet Urban District and Barnet Urban District were abolished and their area was transferred to Greater London to form part of the present-day London Borough of Barnet. At the same time the Potters Bar Urban District of Middlesex was transferred to Hertfordshire.

The highest point in the county is 803 feet (245 m) above sea level, a quarter mile (400 m) from the village of Hastoe near Tring.

As part of a 2002 marketing campaign, the plant conservation charity Plantlife chose the Pasqueflower as Hertfordshire's county flower.

The rocks of Hertfordshire belong to the great shallow syncline known as the London Basin. The beds dip in a south-easterly direction towards the syncline's lowest point roughly under the River Thames. The most important formations are the Cretaceous Chalk, exposed as the high ground in the north and west of the county, forming the Chiltern Hills and the younger Palaeocene, Reading Beds and Eocene, London Clay which occupy the remaining southern part. The eastern half of the county was covered by glaciers during the Ice Age and has a superficial layer of glacial boulder clays.