In vitro toxicology  "In vitro" toxicity testing is the scientific analysis of the effects of toxic chemical substances on cultured bacteria or mammalian cells. "In vitro" (literally 'in glass') testing methods are employed primarily to identify potentially hazardous chemicals and/or to confirm the lack of certain toxic properties in the early stages of the development of potentially useful new substances such as therapeutic drugs, agricultural chemicals and food additives. "In vitro" assays for xenobiotic toxicity are recently carefully considered by key government agencies (e.g. EPA; NIEHS/NTP; FDA), to better assess human risks. There are substantial activities in using in vitro systems to advance mechanistic understanding of toxicant activities, and the use of human cells and tissue to define human-specific toxic effects. Most toxicologists believe that "in vitro" toxicity testing methods can be more useful, more time and cost-effective than toxicology studies in living animals{source needed} (which are termed "in vivo" or "in life" methods). However, the extrapolation from "in vitro" to "in vivo" requires some careful consideration and is an active research area. Due to regulatory constraints and ethical considerations, the quest for alternatives to animal testing has gained a new momentum. In many cases the in vitro tests are better than animal tests because they can be used to develop safer products. The United States Environmental Protection Agency studied 1,065 chemical and drug substances in their ToxCast program (part of the CompTox Chemicals Dashboard) using "in silica" modelling and a human pluripotent stem cell-based assay to predict "in vivo" developmental intoxicants based on changes in cellular metabolism following chemical exposure. Major findings from the analysis of this ToxCast_STM dataset published in 2020 include: (1) 19% of 1065 chemicals yielded a prediction of developmental toxicity, (2) assay performance reached 79%–82% accuracy with high specificity (> 84%) but modest sensitivity (< 67%) when compared with "in vivo" animal models of human prenatal developmental toxicity, (3) sensitivity improved as more stringent weights of evidence requirements were applied to the animal studies, and (4) statistical analysis of the most potent chemical hits on specific biochemical targets in ToxCast revealed positive and negative associations with the STM response, providing insights into the mechanistic underpinnings of the targeted endpoint and its biological domain.