Phenotypic Screening has been the basis for discovery of new drugs before the dawn of molecular biology. A “phenotype” is any type of observable or measurable biochemical or physical characteristic of an individual, organism or complex system: for example, the heart rate of a zebrafish larva or the length of the nerve fibers that grow from a neuron cell in culture. In phenotypic screening, we look at the effect of a substance on a complex system, rather than on an isolated component of a biological pathway. Phenotypic screening can be performed in cells, for example in stem-cell derived human cells, isolated tissues or in whole organisms of varying complexity. While these are the most refined and delicate of all assays, the advantage of phenotypic screening is that it delivers information on the effect of a bioactive compound directly on an advanced disease-relevant parameter.