Optical spectroscopy is a very useful and powerful tool to study complex scatteringmedia, including vegetation, animals and humans.The methods and instrumentspresented in this thesis are based on diffuse reflectance spectroscopy,fluorescencespectroscopy and gas in scattering media absorption spectroscopy (GASMAS).Thework focuses on gas absorption spectroscopy with tunable laser diodes for applicationsrelated to scattering media, while optical reflectance and fluorescence spectroscopy areused as supplementary methods to analyze spectral characteristics.　　Diffuse reflectance spectroscopy and fluorescence spectroscopy are two frequently usedoptical techniques to study chromophores,such as chlorophyll and hemoglobin.In thiswork,the combination of reflectance and fluorescence spectroscopy has been used tostudy the changes of chlorophyll content related to fruit ripening.To quantify thechlorophyll content,two contrast functions based on the absorption peak of chlorophyllat 690 nm and the emission peaks at 685 nm and 740 nm were constructed separatelyfor reflectance and fluorescence spectra.The chromophores in human skin wereinvestigated based on reflectance and fluorescence spectroscopy.The possibility ofapplying optical spectroscopy to evaluate skin bruises was also investigated.Byquantifying the hemoglobin and bilirubin content individually,two contrast functionswere established based on the absorption peak of bilirubin at 470 nm and twoabsorption peaks of hemoglobin at 542 nm and 576 nm.Diffuse reflectancespectroscopy was in this way used to estimate the age of skin bruises.Moreover,reflectance spectroscopy has been used in ecological applications, to distinguish insectsby transforming the spectral shapes to a two-dimensional diagram plotted with twocontrast functions.Fluorescence spectroscopy has shown the possibility to distinguishinsects by the spectral shape,although the spectral information is not so prominent.　　Gas in scattering media absorption spectroscopy (GASMAS) is an optical spectroscopictechnique to study free gas enclosed in porous media.The free gas exhibits narrowabsorption lines and thus its absorption can be isolated from the solids and liquids withmuch broader absorption imprints, although the light has been heavily absorbed.Applications studied in this thesis include gas detection for fruit ripening studies, gasmonitoring inside food packages for quality assurance, fundamental studies for gasdiffusion in ceramics, and gas sensing in the human body for medical diagnostics.Theestimation of fruit ripening was achieved by analyzing the time-dependent oxygensignals related to fruit respiration.The signals were measured successively onpost-harvest climacteric fruits which are characterized by an increase in respiration witha maximum peak.When combined with reflectance and fluorescence spectroscopy,which was used to evaluate the onset day of fruit maturity, the period for fruit ripeningcan be estimated roughly.In addition,GASMAS has been used to detect the oxygencontent in the food packages, including packed milk and bread.Two groups ofmeasurements were performed on milk cartons to demonstrate the temperature effect onmilk storage by placing the packages in the condition of 5 ℃ and 21 ℃.Moreover,water vapor measurement was performed on dough fermentation related to bread bakingto investigate the potential of GASMAS for monitoring the fermentation process.Forapplications of medical diagnostics,GASMAS has been used to measure the watervapor on the frontal sinuses of healthy volunteers to investigate the systematic stabilityand repeatability of the signals.Moreover, a further study was performed to assess theinfluence of decongestant sprays.