Oligonucleotides are short sequences of nucleotides (RNA or DNA), typically with twenty or fewer bases. Automated synthesisers allow the synthesis of oligonucleotides up to 160 to 200 bases. The length of a synthesised base is usually denoted by 'mer' (from 'Greek' meros "part"). For example, a fragment of 25 bases would be called a 25-mer. Oligonucleotides are often used as probes for detecting complementary DNA or RNA because they bind readily to their complements. Examples of procedures that use oligonucleotides are DNA microarrays, Southern blots, fluorescent in situ hybridisation (FISH), and the synthesis of artificial genes. This book brings together leading research in the field from around the globe.