Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. Optically combining one guided sensing beam with a reference beam in an interferometric configuration generates measurable signals. Applying a chemically selective film over the sensing arm of the interferometer provides the basis for a chemical sensor. Tailored chemistries can be passive (e.g.; inducing swelling or dissolution in a film) or active (e.g.; containing reactive or binding sites). Fast and reversible chemistries are the goal, in most cases for both gaseous and liquid applications. Passive mechanisms are used when the target analyte is relatively inert, i.e. aromatic and chlorinated hydrocarbons. Active chemistries developed include tailoring the acid-base strength of the sensing film for pH or ammonia response, and antibody-antigen binding. Currently the integrated optic waveguide platform consists of thirteen interferometers on a 1 X 2-cm glass substrate. A different sensing film deposited on each channel allows for multiple analyte sensing, interferant cancellation, patterned outputs for analyte identification, or extended dynamic range. Sensitivities range from the low ppm to low ppb for both vapor and aqueous applications, 0.01 pH units and ng/mL for biologicals.