Coupling the high specificity of the immunoanalytical reaction with the high sensitivity of optical wave guide light-mode spectroscopy (OWLS) detection gives the possibility to develop immunosensors with in most cases a definitely lower detection limit than traditionally used immunoassays. Measurements were performed on the sensitized surface of optical waveguide grating coupler sensors ( $2400\text{\hspace{0.17em} lines}\mathrm{/mm}$ grating). The OWLS technique is based on the precise measurement of the resonance angle of a polarized laser light ( $632.8\mathrm{nm}$ ), diffracted by a grating and incoupled into a thin waveguide. The effective refractive index, determined from the resonance incoupling angle detected at high accuracy, allows determination of layer thickness and coverage (or mass) of the adsorbed or bound material with ultrahigh sensitivity. OWLS immunosensors were developed as label-free immunosensors with an amino group modified ${\mathrm{SiO}}_2\text{-}{\mathrm{TiO}}_2$ sensor surface on which the immunoreactants could be anchored. One of the components of the antibody–antigen complex was chemically bound on the sensor surface, allowing noncompetitive or competitive detection of the analytes. To illustrate that the resulting immunosensors are suitable for the determination of small and large molecular weight analytes, OWLS sensor formats were applied for quantitative detection of a herbicide active ingredient trifluralin, a Fusarium mycotoxin zearalenone, and an egg yolk protein of key importance in endocrine regulation, vitellogenin.