An arachnoid cyst is a fluid-filled space that is formed between two walls of arachnoid. Those cysts do not communicate with the subarachnoid space. It is congenital malformation of the central nervous system (CNS), which occur with a frequency of 1% of newborns with brain tumour. Arachnoid cyst is a congenital malformation that is formed after the embryogenesis. Currently (the last 20 years), arachnoid cysts are detected during life, since the prenatal period, mainly by imaging techniques. Arachnoid cysts are either “primary”, being a sequelae of faulty embryogenesis of arachnoid, or secondary, or acquired. The latter result from haemorrhages, infections or trauma. Secondary arachnoid cysts commonly communicate with subarachnoid space and, by definitions, are not “true” arachnoid cysts. This review covers definitions and data of the frequency of the arachnoid cyst as well as of current options to detect and diagnose it within the prenatal period since the 13th week of gestation using different modes of ultrasonography and magnetic resonance imaging. The differential for the arachnoid cyst consists of porencephalic cyst, schizencephaly, Dandy-Walker syndrome, arteriovenous malformation of Galen, neoplastic cyst, brain tumours and brain haemorrhages. This review also discusses the prognosis for foetus suffered from arachnoid cysts as well as the role of chromosomal abnormalities in their aetiology. Current recommendations are provided in case of prenatal arachnoid cyst detection.