The objective of this study is to investigate thermo-optical issues in polymer Bragg gratings. The first order thermo-optical model of a polymer Bragg grating is provided. The model was applied to investigate the thermo-optical behavior of the PMMA Bragg grating. The development of a thermo-optical model of an illuminated polymer fiber Bragg grating (PFBG), combining use of the coupled-mode theory with thermal conduction theory and the Transfer Matrix Method (TMM), is presented. The model is verified comparing predicted results with the measured values of the thermo-optical experiment. This model is applied to the prediction of the thermo-optical behavior of an intrinsically-heated and passively-cooled PMMA Fiber Bragg Grating illuminated by a LED and SMLDs and operating over a range of ambient temperatures. Parametric influences on the thermo-optic characteristics and the predictive accuracy of several simplifications in the Bragg Grating relations are also explored.