New technologies have given us the ability to detect genomic variation at resolutions 50-100 times greater than earlier tests. The good news is that we can now detect variations that help explain developmental delays, autism, or multiple congenital anomalies in up to 20% of children. The bad news is that we can also detect small missing or extra pieces of chromosomes that remain unexplained: that is, we don’t know whether they have any clinical significance at all. The rapid pace of technological change may have outpaced the lab’s ability to interpret, providers’ abilities to explain, and patients’ abilities to understand the test results. This Issue Brief summarizes a series of studies examining the uncertainties revolving around chromosomal microarray testing, which has become the new standard of practice in genetic testing of children with unexplained anomalies.