The information in the human genome has already helped scientists identify genes that are responsible for leukaemia and eczema.
In the future it's hoped that the human genome will allow them to prevent many different types of illnesses.
Within the DNA (deoxyribonucleic acid) molecule are 2.9 billion chemical "letters" and certain lists of the letters - in groups of four - make up genes.
Genes make you!
Genes determine your hair colour, race, height and other characteristics of your body.
People get their genes from their parents.
Now scientists from around the world have finished writing the sequence of all the genes in the DNA molecule.
A rough draft of the genome was published less than three years ago, and since then there's been a race to finish the job.
Now it's almost 100% complete - there are a couple of gaps in the decoding, but it's thought to be too costly to fill in the small gaps at the moment.