Disclosed are luciferase polypeptides with improved light-emitting activity and their encoding nucleic acids. These molecules are useful in a range of assays including luciferase-based gene reporter assays, bioluminescence resonance energy transfer assays, protein complementation assays and other applications in which luciferase enzymes are utilized as detectable and/or quantifiable labels. Also disclosed are methods and compositions for increasing the sensitivity and/or improving the kinetics of luciferase-catalyzed reactions as well as decreasing the impact of undesirable variables.