The light guiding properties of optical fibers are the fundament for fiber-optic sensors. The composition of the fiber materials as well as the fabrication methods for both glass optical fibers and plastic optical fibers (POF) are useful knowledge for improvements of the sensor design. A majority of sensing materials includes imbedded luminescent dyes and all O2 fiber-optic sensors are based on O2 quenching of a luminophore. The mechanisms of luminescence and O2 quenching are described. A new procedure for etching a recess in the tip of multimode graded index optical glass fibers was used to improve the mechanical stability of O2 optodes after the sensor chemistry was immobilized into the recess. The recess improved focusing of the excitation light. An O2 optode sensor array was constructed for long-term measurements in soil and sediments. The construction and measuring characteristics of the sensor array system are presented along with a novel approach for temperature compensation of O2 optodes. A method to measure the O2 tension in vivo in the maxillary sinus of cystic fibrosis patients is described. The O2 tension is compared to the bacteria found in the respective sinuses. O2 sensor spots placed inside vials with polymorphonuclear leukocytes revealed strong O2 consumption. The O2 level was measured from outside the vials with a POF. A new method for producing fiber-optic microprobes for measuring scalar irradiance is presented along with an experimental setup for measuring the isotropic response in air and water. The light collecting properties of differently sized scalar irradiance probes (30 μm – 470 μm) produced by the new method were compared to probes produced with previously published methods. A scalar irradiance microprobe was applied to assess the irradiance and total energy dose from different microscopes during the in vitro embryonic development in mouse and pig and the effect on the development was investigated. A highly luminescent cyclometalated iridium(III) coumarin complex was compared to a ruthenium(II) polypyridyl complex or a platinium(II) porphyrin complex. They were all three imbedded in polystyrene spin-coated on cover slips in a 1 μm thick layer. The combination of microscopic spatio-temporal O2 dynamics at the base of heterotrophic biofilms and confocal imaging of biomass and structure demonstrated a complex interaction between biomass distribution, mass transfer and flow. A simple ratiometric intensity based O2 imaging protocol was developed using a conventional digital camera and the O2 distribution images were compared to life-time images obtained using a monochrome fast gate-able CCD camera. The method was applied to a biofilm growth incubator incubated with bacteria occurring in drinking water systems.