The regional museum in Brežice Castle is the most logical place to start if it's your first trip to Posavje - not only does it give you a chance to see the castle itself, but the well-presented and informative exhibits provide visitors with a comprehensive overview of the region in a variety of areas, including history, ethnology, culture and art.

The first part of the collection is arranged chronologically beginning with archaeological finds dating back to some 7000 years, moving through the Bronze and Iron Ages, and finally arriving at the well-represented Romans and early Slavic settlements. Highlights include a large collection of coins and weaponry (much of it coming from the Celtic tribes that once controlled the area), some larger statues and busts, and even several nearly complete skeletons.

The museum's extensive ethnological collection is perhaps its most interesting, especially for those planning to visit vineyards or tourist farms. Dating from the 19th and 20th centuries, the collection consists of hundreds of everyday items used by farmers, winegrowers and various specialised craftsmen. Other exhibitions are dedicated to the peasant uprisings during the 16th century and the Reformation, and contemporary history, which focuses largely on the Second World War.

In the west wing of the castle is the impressive History of Art collection, displaying paintings, sculptures and a handful of other items from the 17th century onwards. Most of the works are by noted Slovene artists, and many of them were formerly owned by Posavje's wealthy aristocratic families. The exhibition begins with the Memorial Gallery of Franjo Stiplovšek, an accomplished painter, graphic artist and the museum's founding director in 1949.