Autism spectrum disorders (ASD) are caused by disruptions in early stages of central nervous system development and are usually diagnosed in first years of life. Despite common features such as impairment of socio-communicative development and stereotypical behaviours, ASD are characterised by heterogeneous course and clinical picture. The most important aetiological factors comprise genetic and environmental influences acting at prenatal, perinatal and neonatal period. The role of rare variants with large effect i.e. copy number variants in genes regulating synapse formation and intrasynaptic connections is emphasised. Common variants with small effect may also be involved, i.e. polymorphisms in genes encoding prosocial peptides system – oxytocin and vasopressin. The environmental factors may include harmful effects acting during pregnancy and labour, however their specificity until now is not confirmed, and in some of them a primary genetic origin cannot be excluded. In several instances, especially with comorbid disorders – intellectual disability, epilepsy and dysmorphias – a detailed molecular diagnostics is warranted, which currently may elucidate the genetic background of disorder in about 20% of cases.