Lethal alleles cause the death of an organism that carry them, but sometimes, death is not immediate - it may take years before death occures, depending on the function of a lethal gene and its impact on an organism. If the mutation results in lethality then the affected gene is essential for growth, development, and survival of an organism. Lucien Cuénot was the first researcher who noticed the effects of lethal genes on mice in 1905, thereafter many researchers started to explore lethal genes. There are several types of lethal alleles due to the fact that they cause death under the different conditions (dominant, recessive, sublethal, conditionally lethal and synthetic lethal alleles). The most aboundant research of the number and the effect of lethal alleles was done on Drosophila melanogaster M. and Mus musculus L., as well as on fish species Lucania goodei G. and Danio rerio H. Comparisons of the data obtained on the different species have provided important conceptions about evolution of lethal alleles, neverthelles the mechanisms of their distribution in different wild populations are still vaguely understood. Effect of some lethal alleles on sex chromosomes, which causes numerous diseases and mutations, was also discovered. Informations about the frequency of lethal alleles in populations are important for understanding population genetics and evolution. Lethal mutations could explain a number of phenomens in evolutionary biology, such as inbreeding and it`s effect on populations. Also, determination of frequency of lethal alleles would enable certain lethal alleles to be correlated with some common hereditary diseases which have a lethal effect. It is therefore very important to continue researching in the area of lethal alleles, in order to get a better insight in their impact on development and life of organisms.