Luciferase assays have a wide range of applications including but not limited to: 

• Study of cell signaling pathways
• Study of RNAi and non-coding RNAs
• Analysis of gene expression
• Analysis of promotor structure
• Analysis of functional SNPs for the promoter
• Detection of the level of cellular ATP 
• Non-invasive gene labeling in live animals

There are several different luciferase assays to choose from including flash assays, glowing assays, single assays and dual luciferase assays. The right one to use will depend on what is needed for the experiment.