The first published paper entitled Birefringent porous silicon membranes for optical sensing presents the results of the investigation on free standing silicon membranes prepared from p-type (110) surface oriented silicon as a material for optical biosensing. The birefringence and sensitivity of this type of porous membrane is theoretically simulated using the Bruggeman model which is extended to incorporate the influence that silicon oxidation has on both birefringence and sensitivity. Using this extended theoretical model a good agreement is found between the measured sensitivity values and the theoretical ones. The final section of this chapter describes the development of a statistical model for characterizing the main depolarization sources responsible of the differences found between the measured transmittance spectra and the theoretical ones. These depolarization sources were mainly the resolution of the spectrometer used, the variations in the porous membrane thickness and the light scattering produced by the pores. The main contribution to the whole depolarization process was found to be the light scattering which increases dramatically with the sample thickness causing an increase in the standard deviation of the measured birefringence values. The second publication, Highly-sensitive anisotropic Porous Silicon based optical sensors, reports on the modeling, fabrication and characterization of PSi membranes from both (110) and (100) silicon was reported. Based on the Bruggeman model the theoretical birefringence and sensitivity was obtained as a function of the porosity and wavelength, with both values have a maximum shown for porosities around 0.5. The impact that the oxidation of pore walls has on birefringence and sensitivity was also studied theoretically. Thereafter a set of PSi samples from different oriented substrates fabricated and characterized. Porous silicon made from (110) shown to be higher values of birefringence than the ones obtained from a (100) surface oriented silicon. Due to the highest birefringence, also the better sensitivity is found in the (110) samples, measuring a value as high as 1407 nm/RIU at a wavelength of 1500 nm. The third publication, Phase sensitive detection for optical sensing with porous silicon, addresses the development of a highly sensitive optical sensor by combining a phase retardation measurement readout scheme for measuring the optical anisotropy, at a wavelength of 1500 nm, with mesoporous silicon membranes fabricated from medium doped n-type (100) surface oriented silicon. These mesoporous membranes with 50 nm pore size are suitable for biosensing applications since they allow the infiltration of the target analytes inside the pores. Birefringence measurements show that the measured birefringence values decrease with sample thickness due to decreasing number of pores that grow in the <113> directions. Subsequent sensing experiments carried out using membranes with thicknesses from 10 to 60 um showed that the amount of depolarized light increases with sample thickness resulting in a poorer resolution of birefringence measurements. Thus, the lowest detection limit obtained was 6.25x10-6 refractive index units, from a 10 um thick membrane. The thermooptic coefficient of that membrane was measured in an aqueous environment and found to be 8x10-4 rad/°C.In the fourth publication, Real-time polarimetric optical sensor using macroporous alumina membranes, we report on the first demonstration of real-time highly sensitive optical sensing using free standing alumina membranes of 200 nm pore diameter. In comparison with the previously studied porous silicon membranes, these macroporous alumina membranes not only allow the infiltration of biomolecules but also present flow-through properties so that analytes can be delivered fast to bioreceptors placed on the surface of the pores throughout their length. The birefringence measurements of this type of membrane at wavelengths of 1500 nm, 980 nm and 808 nm show a decrease at shorter wavelengths where depolarization caused by light scattering becomes more significant. The volumetric sensing experiments performed at the same wavelengths demonstrated detection limits of in the range of 10-6 refractive index units. The lowest detection limit was 5.2x10-6 refractive index units measured at a wavelength of 980 nm which demonstrates the possibility of developing low-cost multiplexed devices through the use of silicon CCD detectors. The last publication, Real-time polarimetric biosensing using macroporous alumina membranes, describes the immobilization of the allergen protein ß-Lactoglobulin by means of functionalizing the alumina membrane pores surface with an epoxysilane. The specific binding response produced by the recognition of the rabbit anti-ßlactoglobulin antibody by the immobilized protein is then measured in real-time by the developed porous alumina based optical biosensor microsystem. The binding response from the recognition between this first antibody and a secondary antibody anti-rabbit IgG was then also measured in real-time demonstrating the biosensing capabilities of the free-standing membrane when its birefringence is measured using an polarimetric setup.