Epigenetic modifications are alterations in the cell nucleus that affect gene expression and can occur in chromatin at the level of DNA methylation or histone modifications. Such ‘epigenetic marks’ can be heritable through cell division but leave the DNA sequence unchanged. Post-­translational modifications can be found on the histone proteins associated with DNA; the majority of histone modifications are found on the lysine-­rich N-‐terminal amino acid “tails”. Histone acetylation and methylation influence the chromatin structure by loosening or tightening the packaging of DNA, respectively, in association with other chromatin modifiers. Condensed chromatin is linked to transcriptional silencing and genetic imprinting and also occurs at chromosomal centromeres, where it is linked to kinetochore binding. Heart development is well studied, but the epigenetic processes involved are not yet completely understood. While active chromatin mechanisms such as histone acetylation and chromatin remodelling have been described in the heart, the role of gene repressive epigenetic mechanisms has been poorly investigated. Cardiomyocytes are post-­mitotic cells that do not divide to regenerate a damaged heart. The regeneration of cardiomyocytes after myocardial infarction is an important topic of interest in cardiovascular science. There are various approaches to heart repair after infarction, including activating cardiomyocytes so they become mitotic once again, or growing cardiomyocytes in vitro to attach to a lesion site. An important factor in these approaches is understanding the epigenetic mechanisms controlling cell division. In this thesis, we aim to advance the current knowledge of the epigenetic repressive mechanisms involved in cardiomyocyte formation and heart development to explain their lack of regenerative capacities. We studied the epigenetic changes that occur during cardiac development leading to a non-­‐regenerative state to pinpoint the moment at which these changes arise. We found that the epigenetic process is independent of whether cardiac lineage differentiation occurs during embryogenesis or during differentiation in vitro. We discovered that cardiac heterochromatin displays a singular epigenetic signature during development as compared to brain, another post-­mitotic tissue, or liver, an actively regenerative tissue. We observed an epigenetic change in the repressive histone modification histone H3 lysine 9 trimethylation that was specific to heart development. This change involved a nuclear reorganisation of heterochromatin and a reduction of the levels of this mark in E13.5 and E14.5 embryos, as compared to E10.5 embryos. This was consistent with our observations of the histone lysine methyltransferase SUV39H1, the levels of which were lower after stage E10.5 of development. However, contradictorily, in differentiated cardiomyocytes in vitro, SUV39H1 was increased but showed low levels of H3K9me3, compared to ES cells, which had low levels of SUV39H1 and high levels of H3K9me3. We detected extremely low levels of the H3K9me3 in adult heart tissue. We observed that in adult hearts, the myocardium had maintained these major changes in H3K9me3, while this effect was not observed in the epicardium. Genomic studies were carried out to determine changes at a genomic level between the two key epigenetic stages in heart development we identified at E10.5 and E13.5. Methylated DNA immunoprecipitation sequencing and chromatin immunoprecipitation sequencing for H3K9me3 analyses were carried out to find overall changes in methylation patterns. No global changes in DNA methylation were detected between these developmental stages. These results imply that the differences observed in H3K9me3 are due to remodelling of the heterochromatin during heart development and cardiomyocyte formation, rather than quantitative changes.