Aptamers are oligonucleotides (single-strand RNA or DNA) with three-dimensional shapes that can bind to their target with conformational changes. Recently, aptamers have attracted increased attention in analysis because they have all the advantages of antibodies, as well as unique merits, such as thermal stability, small size, reversible binding, low cost, and easy signal transduction. In this chapter, several types of aptamer selection processes, which can be categorized into two groups (SELEX (systematic evolution of ligands by exponential enrichment) strategies and SELEX variant strategies), are explained in detail. Furthermore, we briefly review various applications of aptamers for the diagnosis of diseases, drug release, antimicrobial activity, bioanalysis, and serotyping.