Intellectual and developmental disorders (IDD) are one of the main reasons for referral in pediatric, child-neurological and clinical genetic service. It is a variable and heterogeneous manifestation of central nervous system dysfunction, affecting approximately 2-3% of the population. A significant progress in identifying the underlying mechanism/genetics was first made with the implementation of array based technologies to screen the genomes of patients for chromosomal aberrations. Recently whole exome sequencing (WES) has been implemented in the clinical settings further increasing the diagnostic yield in this group of patients with approximately 30%. However, the etiology is still unknown in 40-50% of these patients The aim of this project is to use whole genome sequencing (WGS) in healthy parent-offspring trios to identify novel causative genes in IDD. By using WGS, variants located in regions not covered by WES can also be studied, as well as copy number variants (CNVs), chromosome inversions and translocations. By using this strategy, WGS has the capability to replace both WES, chromosomal microarrays (CMA) and karyotyping in clinical settings. Additionally, smaller CNVs, to small to be detected by CMA can also be visible. Data from this study will not only be used to detect novel IDD genes but also to implement the methodology into clinical diagnostics. All patient samples and data used in this study have been decoded, Ethical approval has been received from the ethical review board and informed consent has been obtained from the parents of all patients.