Upon admission to intensive care units (ICU), the differential diagnosis of almost all infants with diseases of unclear etiology includes single locus genetic diseases. Rapid whole genome sequencing (rWGS), including sample preparation, short-read sequencing-by-synthesis, informatics pipelining, and semiautomated interpretation, can now identify nucleotide and structural variants associated with most genetic diseases with robust analytic and diagnostic performance in as little as 13.5 h. Early diagnosis of genetic diseases transforms medical and surgical management of infants in ICUs, minimizing both the duration of empiric treatment and the delay to start of specific treatment. Both positive and negative rWGS tests have clinical utility and can improve outcomes. Since first described 10 years ago, rWGS has evolved considerably. Here we describe our current methods for routine diagnostic testing for genetic diseases by rWGS in as little as 18 h.