Epigenetic modifications can alter the function of genes. The epigenetics changes are caused by environmental effects, which lead to chemical modifications of the DNA or the chromatin. The mechanisms involve the influence of small interfering siRNAs on gene silencing. Epigenetic changes normally last only during the life-time of an individual and are erased in embryos and eggs for a naive progeny. The genomes are reprogrammed and the chemical modifications removed to restart the next generation. However, there are mechanisms that allow the genome to escape from such a clearing effect so that modifications can be transmitted to one or more subsequent generations. In the germline of animal cells small RNAs, including piRNAs, have evolved which guarantee a higher degree of fidelity for transmission of genetic information, guarding especially against the detrimental effect caused by transposon activity. piRNA is essential for transposon silencing for survival of a species and protection of subsequent generations. Inactivation of piRNA results in abundant transposon activity and sperm infertility. The effect in humans has been described but is less distinct. Some stress-induced epigenetic changes are transitory in mice and can be reversed by a change of environment or lifestyle.