The embryonic stem cell test (EST) is a high-throughput in vitro screening assay for developmental toxicity free of animal use. The EST uses the ability of murine embryonic stem cells to differentiate into the mesodermal cardiac lineage in combination with two cytotoxicity test systems. Validation of the EST showed that the test system is very promising as an alternative method to animal testing, however to optimize predictability and increase knowledge on the applicability domain of the EST, improvements to the method were proposed and studied. In this review we discuss the first definition of the EST followed by the innovative approaches which have been proposed to increase the predictivity of the EST, including implementation of molecular endpoints in the EST, such as omics technologies and the addition of alternative differentiation models to the testing paradigm, such as neural and osteoblast differentiation and the use of human stem cells. These efforts to improve the EST increase the value of embryonic stem cells used as in vitro systems to predict developmental toxicity.