The geography of the regional unit of Kilkis is characterized by the wide and flat Axios river valley in the westcentral part, and mountain ranges on its western and northeastern edges. The mountain range in the west, on the border with Pella regional unit, is Mount Paiko (highest peak 1,650 m or 5,413 ft). In the north, the Kerkini range straddles the border with North Macedonia. At 1,874 m or 6,148 ft the highest peak in Kilkis regional unit is located here. The border with Serres regional unit to the northeast is formed by the lower Kroussia range (highest peak 1,179 m or 3,868 ft). Lake Doirani is situated in the north, shared with North Macedonia. Kilkis borders the Thessaloniki regional unit to the south.
The area of the modern regional unit was part of the Kingdom of Macedonia from the 8th century BC until the Third Macedonian War (171 BC - 168 BC), when it became a part of the Roman Empire. At the division of the Roman Empire in 395 AD, the area joined the eastern part, later known as the Byzantine Empire. Between the 7th century and the 11th century, it changed hands between the Byzantine Empire and the Bulgarian Empire repeatedly. In the 13th and 14th century Western Europeans and Serbs briefly ruled the area. The Ottoman Empire conquered the area in 1371, and ruled it until the First Balkan War of 1912. In the Second Balkan War of 1913, the Greek army captured the area, which became part of Greece. It absorbed many of the Greeks from what is now the Republic of North Macedonia, especially from Gevgeli, Vogdantsa, Polyane and Stromnitsa.[2]
Until 1939, when created into a separate prefecture, the area was part of the Thessaloniki Prefecture.[3] At the 2011 Kallikratis reform, the Kilkis Prefecture became a regional unit.
The Motorway 1 (E75, Skopje - Polykastro - Thessaloniki - Athens) runs through the regional unit from north to south. Two railways pass through the regional unit: from Thessaloniki to Skopje via Polykastro and Idomeni, and from Thessaloniki to Sofia, Istanbul and Alexandroupoli via Kilkis and Mouries.
Administration
The regional unit Kilkis is subdivided into 2 municipalities. These are (number as in the map in the infobox):[4]
As a part of the 2011 Kallikratis government reform, the former Kilkis Prefecture (Greek: Νομός Κιλκίς) was transformed into a regional unit within the Central Macedonia region, without any change in boundaries. At the same time, the municipalities were reorganised, according to the table below.[4]