It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover all ancient knowledge. The work's subject area is thus not limited to what is today understood by natural history; Pliny himself defines his scope as "the natural world, or life". It is encyclopedic in scope, but its structure is not like that of a modern encyclopedia.
The work is divided into 37 books, organised into ten volumes. These cover topics including astronomy, mathematics, geography, ethnography, anthropology, human physiology, zoology, botany, agriculture, horticulture, pharmacology, mining, mineralogy, sculpture, painting, and precious stones.
Pliny's Natural History became a model for later encyclopedias and scholarly works as a result of its breadth of subject matter, its referencing of original authors, and its index. The work is dedicated to the emperor Titus, a son of Pliny's close friend, the emperor Vespasian, in the first year of Titus's reign. It is the only work by Pliny to have survived, and the last that he published. He began it in 77, and had not made a final revision at the time of his death during the AD 79 eruption of Vesuvius.
Pliny's Natural History was written alongside other substantial works (which have since been lost). Pliny (23–79) combined his scholarly activities with a busy career as an imperial administrator for the emperor Vespasian. Much of his writing was done at night; daytime hours were spent working for the emperor, as he explains in the dedicatory preface addressed to Vespasian's elder son, the future emperor Titus, with whom he had served in the army. As for the nocturnal hours spent writing, these were seen not as a loss of sleep but as an addition to life, for as he states in the preface, Vita vigilia est, "to be alive is to be watchful", in a military metaphor of a sentry keeping watch in the night. Pliny claims to be the only Roman ever to have undertaken such a work, in his prayer for the blessing of the universal mother:
Hail to thee, Nature, thou parent of all things! and do thou deign to show thy favour unto me, who, alone of all the citizens of Rome, have, in thy every department, thus made known thy praise.