This work concerns planar optical waveguide sensors for biosensing applications, with the focus on deep-probe sensing for micron-scale biological objects like bacteria and whole cells. In the last two decades planar metal-clad waveguides have been brieflyintroduced in the literature applied for various biosensing applications, however a thorough study of the sensor configurations has not been presented, but is the main subject of this thesis. Optical sensors are generally well suited for bio-sensing asthey show high sensitivity and give an immediate response for minute changes in the refractive index of a sample, due to the high sensitivity of optical bio-sensors detection of non-labeled biological objects can be performed. The majority of opticalsensors presented in the literature and commercially available optical sensors are based on evanescent wave sensing, however most of these sensors can only detect substances in the close vicinity to the sensor surface limited to below 200 nm. Bacteria andwhole cells with sizes of micron-scale exceed that range and thus sensors suitable for deepprobe sensing are important. The present thesis deals with optical, planer waveguide sensors for deep-probe evanescent wave sensing with emphasis on detection ofmicron-scale biological objects. I have demonstrated reversesymmetry operation using the simplest possible configuration, a freestanding glass plate acting as a multimode sensor [N. Skivesen et al, Opt. Lett. 28, 2003]. Such a sensor has only beenrealized by Qi et al [Zm Qi et al, Sens. Actuators B 81, 2002] before, however the sensing principle we present results in a broad detection range from gasses to solid materials and is different from the principle suggested by Qi et al with a highlylimited detection range. Metal-clad waveguide sensors are suitable for deep-probe sensing and can be operated in two different modes. I have made a thorough optimization of the waveguide parameters of both MCWG-types in terms of sensitivity [N. Skivesenet al, Sens. Actuators B 106, 2005] and [N. Skivesen et al, Opt. Lett. 30, 2005]. Each of the sensor types have previously been realized by Salamon et al [Z. Salamon et al, Biophys. J. 73, 1997] and Zourob et al [M. Zourob et al, Sens. Actuators B 90,2003] and presented for biosensing in connection with experiments, however no thorough study of the impact of the various parameters involved in the sensor configurations have been presented. The studied sensor configurations have been fabricated andapplied for refractive index measurements and cell-detection, thus this thesis also includes presentation and analysis of experimental setups, descriptions of fabrication processes and measurements with all three sensors, where single cell detection isshown by use of the metal-clad waveguide sensors.