Clinical and environmental analyses often require immunochemical detection and quantification of small molecules (haptens) that are available as biomarkers. However, the affinity ceilings of conventional anti-hapten antibodies, which are produced by immunizing animals, prevent subfemtomole-range determinations with competitive immunoassay formats. "Sandwich-type" noncompetitive (immunometric) assays allow for sensitive determinations of macromolecules (subattomole-range) and the direct relationship between analyte amount and signal intensity provides higher accessibility to modern high-throughput sensing systems. Unfortunately, sandwich-type assays require that analytes have at least two epitopes, and thus are not applicable to haptens. Antibody engineering, i.e., genetic manipulation of antibody molecules, could provide artificially improved reagents that enable us to overcome these limitations. In this review, we summarize recent successful developments and applications of engineered antibodies for sensitive and high-throughput hapten sensing.