The standard genetic code forms a nearly universal set of rules that translate the nucleotide sequence into the 20 naturally occurring amino acids that build proteins. However, in nature, there is a large number of amino acids that are not proteinogenic, and, if a way of incorporating them is found, it could give proteins numerous new characteristics and functions. Advances in science and the development of synthetic biology and xenobiology enabled the "recoding" and development of semi-synthetic and synthetic organisms with altered or alternative genetic code. For that scientists have used various methods and models such as suppressor tRNAs, synthetic base pairs, quadruplets instead of nucleotide triplets, mutated ribosomes, etc. Today, most research is based on how the expansion of the genetic code and proteins with new properties affect processes such as transcription, translation, and post-transcriptional modifications of proteins and the potential application of such organisms in genetic engineering, biotechnology, and medicine. This bachelor thesis presents a brief overview of the methods used by scientists to expand the genetic code, description of the organisms that have been recoded so far, and their potential application in science and biotechnology.