This systematic review sought to document the corpus of language intervention research with low-SES children and help the field move forward. Low-SES children (birth to 8) are at increased risk for infrequent language input from parents (30-Million Word Gap), delays in vocabulary/language, readiness for school, and later school problems. From a repository of 1494 articles published between 1980 and 2016, we identified 513 rigorous intervention reports. We reviewed the 140 studies that included low-SES children in terms of ecological validity, trustworthiness, and readiness for scale-up given a future goal of population-level prevention. Results indicated a relatively high degree of trustworthiness, a moderate degree of ecological validity, and a low degree of readiness for scale-up. All studies used experimental or quasi-experimental designs, the majority being the gold standard, randomized-control trial. Parents and/or other caregivers in authentic settings were the intended intervention implementers (ecological validity). However, research staff implemented the majority of interventions. Most studies were early- to mid-stage, none directly investigated scale-up. Weaknesses in scale-up readiness were infrastructure (i.e., manuals), community engagement (i.e., social validity), and progress monitoring/evaluation (i.e., technology), among others (i.e., cost). We identified 37 areas of improvement in future research with implications for preventing the Word Gap. (author abstract)