What is the story behind the term “germ cell epigenetics”, what can we learn from the changes in methylation during gametogenesis, and how could it be put to good use? This thesis aims to explain how changes in the DNA methylation affects the development of primordial cells into mature sperm. Methylation is the epigenetic process controlling what genes will be expressed and what genes will be silenced. Epigenetics can be viewed as the environmental inheritance we get from our ancestors. It reflects changes that occur due to normal aging (development) but also environmental influences. Alarmingly, our epigenome reflects not only our lifestyle choices but also the experiences of our parents and perhaps also grandparents. Some methylation processes are inherited from the father and some from the mother, part of which reflects the imprinting process. Today, it is known that some genes in specific sites will be maternally imprinted and others paternally imprinted, making each parent contribute to various specific cells. This is all done by the process of methylation. The development of the oocyte and sperm starts in the embryonic period and varies drastically. Male developing germ cells show changes in their methylation process already during the fetal period and continue during spermatogenesis. Sperm of infertile males have been shown to carry methylation defects and there is a danger of passing the methylation errors to the next generation. Thus, understanding the male germ cell methylome can help us understand its vulnerabilities, as well as identify factors that may influence its methylation. I hope that by learning more about these processes, we will be able to understand ourselves better and create better diagnostic and treatment options for men with infertility problems, as well as help us create personalized preventive approaches for those problems.