Aptamers are short single-stranded oligonucleotide (DNA or RNA) molecules with the ability to bind to other molecules with high affinity and specificity. Nowadays, aptamers are used as biosensing molecules with broader applications, such as ribozymes (RNA enzyme molecules with catalytic activities); riboswitches (modulating translational activities) and ligands, recognizing specific target binding. Aptamers are folded, directed by their sequences, acquiring specific 3D conformations. This 3D conformation property has been widely applied in order to recognize cell structures. Aptamers evolve from random oligonucleotide pools by a process called Systematic Evolution of Ligands by Exponential enrichment (SELEX). Conceptually, the SELEX process is controlled by the ability of these small oligonucleotides to fold into unique 3D structures that can interact with a specific target with high specificity and affinity [1]. This is a dynamic process, repeating exposures and elutions, allowing to screen in a heterogenic pool in low concentration ranges. A standard SELEX method has four major steps: 1.- Exposure of random sequence single stranded nucleotide oligo library to a target. 2.- Binding of oligos to the target molecule. 3.- Selection of binders and removal of non-binding oligos. 4.- Amplification of the binder fraction and portioning of the amplicon to single strand. These steps are iteratively performed till a pool of high binding aptamer is screened out from the library. The oligonucleotide library consists of a random base-sequence flanked on both ends by primer binding sites, which aids in amplification and enrichment (Safeh et al., 2010; Kim et al., 2013). Here is described how SELEX have been applied to develop aptamers, recognizing specific outer membrane structures, as LamB.