Anabolic androgenic steroids (AAS) are frequently abused in human and animal sports as performance-enhancing drugs, and consequently their use is controlled by internation- al sports authorities. Testosterone is one of the most frequently used AAS, and therefore the accurate determination of its levels in biological fluids is very important. The authors de- scribe the selection of testosterone-binding aptamers per- formed using a classic SELEX approach with the target immobilized on magnetic beads. Counter selections with structurally similar steroids were implemented at different stages. Pools from different selection rounds were sequenced with Next Generation Sequencing and ten aptamer candidates were selected for further characterization. Low nanomolar range dissociation constants were calculated by a bead-based PCR assay and verified by microscale thermophoresis. Future work will focus on the development of aptamer-based plat- forms for the sensitive detection of testosterone in biological samples and the validation of these assays for the rapid screen- ing of suspicious samples.