Collaboration for Down Syndrome Progress (CDP)

Inclusion Criteria: * Individual with Down syndrome To be considered potentially eligible for this CDP, a participant must meet the following criteria: * Diagnosis of Down syndrome (with the exception of control participants for the Subsample study on Imaging). We will be enrolling participants with all types of Down syndrome including standard Trisomy 21, mosaic Down syndrome, and translocations. * Primary language is English, Spanish, or Portuguese. Support Person * Able to attend in-person or remote visits. * Able to provide accurate information about the study participant's clinical outcomes and family history. * Primary language is English, Spanish, or Portuguese. Biological parent(s) biospecimen collection * Biological parent of the enrolled participant. * Willing to provide a biological sample. * Primary language is English, Spanish, or Portuguese. Imaging subsample study controls * A subset of controls will be enrolled to match a cohort of individuals who take part in the CDP Subsample Study on imaging. Healthy control infants must be born at greater than 36 weeks gestational age and must have at least one older sibling. The sibling criterion allows comparability for potential analyses combining control data from DS-CDP with analogous control data previously collected by IBIS for other neurodevelopmental studies. Exclusion Criteria: * A participant will also be excluded if a healthcare professional determines that CDP involvement poses a risk of mental and physical harm to the participant. Imaging subsample study exclusion criteria for individuals with Down syndrome and controls: * Known genetic conditions or syndromes with effects on neurobehavioral development (except for Down syndrome) such as Fragile X syndrome, Williams syndrome, or Prader-Willi syndrome. We may also exclude other syndromes such as Marfan's syndrome or Turner syndrome because of overall significant multisystem effects. * Birth weight \< 2,000 grams or gestational age \< 34 weeks (infants with Down syndrome infants) or \<37 weeks (control subjects) * Significant perinatal adversity, in utero neurotoxin exposure or maternal gestational diabetes requiring medication management * Significant medical conditions (unrelated to Down syndrome) affecting growth, development, cognition or sensory impairments. We will conduct a review of the medical history to identify major medical conditions that might be a reason for exclusion. * Neurological event like a stroke * Congenital infection associated with altered development (e.g., congenital rubella) * Significant infection affecting the brain after birth, like meningitis * In infants with Down syndrome, severe medical issues which may exert significant developmental effects beyond Down syndrome such as: * Cyanotic cardiac abnormalities (e.g., Tetralogy of Fallot) that affect overall oxygen levels * Frailty because of recovery from significant surgery or extended hospital stays * Contraindication for MRI * English not predominant home language * Family history of a first-degree relative with psychosis or bipolar disorder (controls only) * To be consistent with prior imaging studies from the infant brain imaging study, healthy controls will additionally be excluded for a family history of a first- or second-degree relative with ASD to also allow them to serve as a comparison group for elevated familial liability for ASD. Sleep subsample study exclusion criteria: * Participants under the age of 12 who are currently being treated for obstructive sleep apnea using a positive airway pressure (PAP) device. * For the WatchPAT device, participants must weigh more than 65 pounds. They cannot have a permanent pacemaker or sustained non-sinus cardiac arrhythmias.