When ordering their oligos, most scientists wonder: “What level of purity do I need?” This chapter will help you make the right choice, based on clear information with respect to the purification methods available from Eurogentec and the specifications (yields, purity levels, ...) of the corresponding oligonucleotides. The initial aim of any purification method is to remove the by-products resulting from the removal of the protecting groups and other synthesis by-products. As with the purification of other biomolecules, this removal of small (in)organic impurities is referred to as “desalting”. The next goal is to enrich the final product in full-length oligonucleotides. Although extremely efficient (Eurogentec has a typical synthesis step yield of 98-99 %), about 1-2 % of all extensions fail. This phenomenon leads to truncated elongation products, the longest of which is (n-1) mer. The relative proportion of undesired (n-x) material increases as oligo length increases. In general, the purity level required for a specific application depends on the potential problems which may occur as a result of the presence of truncated sequences.