Acutely ill infant inpatients who have an undiagnosed illness, and their families, will be eligible to participate in the study. The investigators will enroll up to 1,000 infants. Locally, the study population will be recruited from Rady Children's Hospital (RCH) inpatient population, primarily the neonatal intensive care unit (NICU), pediatric intensive care unit (PICU), and cardiovascular intensive care unit (CVICU), with a smaller population presenting to other hospital in-patient services. Recruitment will be targeted at the RCH main campus, but it may include referrals from satellite locations in the RCH network (particularly the RCH NICU network throughout San Diego County). All patients will continue to receive routine care as clinically indicated, including the state newborn screen and other genetic testing as determined by their treating providers. Half of the affected study participants will be randomized to receive rapid whole genome sequencing (WGS) and the other half will receive rapid whole exome sequencing (WES). Each arm will initially be analyzed using the patient's (proband's) sample only. If a proband-only analysis fails to yield a diagnosis, genomic data from the biological family members (typically parents), when available, will be used to supplement analysis (trio analysis). Occasionally, a second affected sibling may be available for family analysis. Not infrequently, the father is not available for study. Similarly, the investigators anticipate the need for targeted genetic analysis of biological parents, and possibly other family members, to confirm diagnostic results and/or provide additional information regarding inheritance.