This　chapter　deals　with　the　fundamental　of　the　photoluminescence　(PL)　spectroscopy　and　its　applications　to　study　the　chemical　reaction　of　molecules　on　solid　surfaces　and　the　reactivity　of　various　heterogeneous　solid　catalysts　in　relation　to　their　properties　in　adsorption,　catalysis,　and　photocatalysis.　After　a　short　introduction,　the　basic　principles　of　PL　spectroscopy　are　explained　in　relation　to　the　definitions　of　fluorescence　and　phosphorescence.　And,　the　PL　features　of　the　semiconducting　catalysts　are　discussed　in　relation　to　the　surface　band　structures.　Next,　the　practical　aspects　of　static　and　dynamic　PL　with　the　spectral　parameters　including　wavelength　and　spectral　shape,　lifetime　and　the　Stern-Volmer　expression,　energy　transfer　and　migration,　and　ultrafast　time-resolved　PL　spectroscopy　are　discussed.　In　Sect.　14.4,　which　is　one　of　the　cores　of　this　chapter,　the　characterization　of　the　catalytically　active　sites　by　applying　in　situ　PL　spectroscopy　are　discussed　with　various　single-sites　heterogeneous　catalysts　such　as　Ti-oxide,　V-oxide,　and　Mo-oxide　single-site　containing　catalysts,　and　carbon　containing　catalysts　such　as　polymeric　carbon　nitride.　In　Sect.　14.5,　characterization　of　acidic　and　basic　surface　sites　is　discussed　by　means　of　luminescence　probe　molecules　and　in　situ　PL　spectroscopy.　In　Sect.　14.6,　in　situ　PL　studies　are　discussed　in　relation　to　the　photocatalytic　reaction　processes　on　inorganic　and　organic　semiconducting　catalysts.　Especially,　photo-generation　of　electron　and　holes,　their　lifetimes,　and　reaction　dynamics　are　discussed.　In　Sect.　14.7,　effects　of　temperature　on　PL　spectra　of　their　intensity　and　wavelength　are　discussed.　In　Sect.　14.8,　effect　of　magnetic　fields　on　PL　spectra　are　discussed.　Section　14.9　is　the　conclusion　and　outlook　of　the　PL　spectroscopy　in　catalysis　and　photocatalysis.　©　2023,　Springer　Nature　Switzerland　AG.