Outcomes of treating gestational diabetes mellitus (GDM) are not well-established. To summarize evidence about the maternal and neonatal benefits and harms of treating GDM. 15 electronic databases from 1995 to May 2012, gray literature, Web sites of relevant organizations, trial registries, and reference lists. English-language randomized, controlled trials (n = 5) and cohort studies (n = 6) of women without known preexisting diabetes. One reviewer extracted data, and a second reviewer verified them. Two reviewers independently assessed methodological quality and evaluated strength of evidence for primary outcomes by using a Grading of Recommendations Assessment, Development and Evaluation approach. All studies compared diet modification, glucose monitoring, and insulin as needed with no treatment. Women who were treated had more prenatal visits than those in control groups. Moderate evidence showed fewer cases of preeclampsia, shoulder dystocia, and macrosomia in the treated group. Evidence was insufficient for maternal weight gain and birth injury. Low evidence showed no difference between groups for neonatal hypoglycemia. Evidence was insufficient for long-term metabolic outcomes among offspring. No difference was found for cesarean delivery (low evidence), induction of labor (insufficient evidence), small-for-gestational-age neonates (moderate evidence), or admission to a neonatal intensive care unit (low evidence). Evidence is low or insufficient for many outcomes of greatest clinical importance. The strongest evidence supports reductions in intermediate outcomes; however, other factors (for example, maternal weight and gestational weight gain) may impart greater risk than GDM, particularly when glucose levels are modestly elevated. Treating GDM results in less preeclampsia, shoulder dystocia, and macrosomia; however, current evidence does not show an effect on neonatal hypoglycemia or future poor metabolic outcomes. There is little evidence of short-term harm of treating GDM other than an increased demand for services.