Pluripotent human embryonic stem cells (hESC) have the potential to differentiate to any cell type of the human body. This characteristic has sparked researchers to study the use of hESC for regenerative medicine, drug screenings and embryonic development. We have recently optimized differentiation of hESC to cardiomyocytes, including growth factor directed differentiations as monolayers or as three-dimensional aggregates (embryoid bodies or EBs). Previously, we have demonstrated that hESC-derived cardiomyocytes (hESC-CM) faithfully recapitulate the early molecular events during embryonic development. Recently, we have generated a cardiac reporter line by introducing Green Fluorescent Protein (GFP), in the genomic locus of the early cardiac transcription factor NKX2-5, which enables us to visualize the derivation of NKX2-5+ cardiomyocytes during in vitro differentiation and purify these cells by Fluorescent Activated Cell Sorting (FACS). The combination of different transcription factor-coupled fluorescent reporters in this so-called “rainbow” hESC cell line, covering sequential stages of the cardiac lineage, will allow us to identify and characterize pathways for specific subtypes of the cardiac lineage at early and later stages during differentiation. Furthermore, a better understanding of these developmentally related processes will be further important for progress in fields of tissue engineering, disease modelling, drug toxicity and discovery, which most likely will lead to improved tailor-made therapies and better and safer medicines on the market.