A molecule of deoxyribonucleic acid (DNA) stores information characteristic and necessary for the organism at every level of life organization, i.e. individual, population and species. As the molecule is transferred from generation to generation, the DNA template is the most important and superior to other structural and functional entities. Coding sequences (within genes) as well as noncoding sequences (outside the genes) remain the source of information about an individual's phenotype even post mortem. Deciphering sequences of ancient DNA (aDNA) templates provides the opportunity to identify physiological as well as pathological traits of the individual studied, that are either confirmable or impossible to detect with archaeological measurements. These include the origin of the analysed individual in the phylogenetic sense, his relationship to other individuals unearthed in the neighbourhood, predispositions to hereditary as well as infectious diseases, even skin colour or the ability to speak. Moreover, a comparison of characteristic species sequences, and in the near future whole genomes, gives insight into evolutionary changes, and thus possible regulatory mechanisms. The information which is acquired from fragments of aDNA (sometimes very short), and translated into the language of population genetics, may offer surprising data. The most important are: impact of sudden and slow climate changes on new species formation, cause of their extinction or even characteristic behaviour of the species. Although aDNA research was launched only two decades ago, rapid methodological progress in the field is observed due to growing knowledge of DNA chemical degradation. More and more sophisticated analytical procedures have made DNA retrieval possible, which is especially important in the case of extinct species. Modification patterns of the chemical structure of DNA are reviewed in the paper. Degradation processes, which can significantly alter the authentic information encoded within the analysed sequence, are not the only obstacle encountered by an aDNA researcher. High risk of sample contamination with contemporary DNA needs to be considered as well. Special patience should be taken when the DNA of humans is analysed. Ubiquitous human sequences present in the surroundings can affect analysed samples at every step of the procedure, thus falsifying the obtained data. Therefore, a number of both simple and more sophisticated precautions should be undertaken by researchers, including the stage of archaeological sampling, collection, DNA isolation and analysis - in order to avoid contamination with modern molecules. A few methods commonly applied to authenticate the results as well as interpret the obtained data are discussed. Diverse applications of the analysis of historical templates were described in the literature, some of them, representative of different fields of interest (including evolution) have also been reviewed. These include the most recent and spectacular DNA sequencing of our closest relative, i.e., Neanderthal, as well as that of two extinct animals - the mammoth and the cave bear.