Probiotics, live microorganisms conferring a health benefit on the host when given in appropriate amounts, have been widely used in humans, including neonates, to promote gut health by limiting pathogen growth, promoting enterocyte differentiation, downregulating intestinal inflammation, and improving mucosal barrier integrity. In neonates, despite numerous studies and discussions for nearly 3 decades, there is still uncertainty about routine supplementation of probiotics in preterm infants. A 2020 Cochrane review1 concluded that large, high-quality randomized clinical trials (RCTs) are needed to provide sufficient evidence. In this Viewpoint, we summarize the sources of uncertainty in assessing the benefits and harms of probiotics for the prevention of necrotizing enterocolitis (NEC), describe the current options for clinicians, and discuss the theoretical influence that future trials may have on treatment effect estimates for probiotic supplementation.