Lastovo and its archipelago are made up of Jurassic and Cretaceous sedimentary rocks consisting of mainly dolomite, dolomitic limestone and pure limestone.
Hum (417m), Lastovo's highest hilltop, offers the most beautiful view of Lastovo's intricate landscape with rounded hills, fields, and bays and coves cutting deeply into the coastline on the one hand, and of the archipelago and endless sea on the other. The second- and third-highest hilltops are Mali Hum (415 m), and Pleševo Brdo (400 m).
Morphologically significant is the interplay of numerous wooded hills and karst valleys – fields (the largest of them being Vinopolje, Lokavje and Prgovo), formed in the impermeable dolomite beds or semipermeable dolomitic limestone beds.
Lastovo's karst structure was also beneficial to the formation of caves. The most famous of them is Rača Cave in the southeastern part of the island. It was protected as a geomorphological natural monument and archaeological site in 1965. Another famous one, Medjedina Cave, used to be the habitat of the Mediterranean monk seal. Today, it houses large colonies of bats.
The Lastovo archipelago is divided into several groups of islands:
- Škoj od Mrčare
- Bratin otok
- Škoj od Kopišta
- Karlovića tovari
- Mladine (Saplun)
- Arženjak Mali i Veliki
- Veliki Golubinjak
- Tajan Mali i Veliki
- Crna i Bijela
- Gornji Vlašnik and Srednji Vlašnik
- Mrkjenta pod Glavat
- Mrkjenta pod Smokvicu
- Sestrice (Veja, Mala)