The measure of the promoter activity has a broad meaning. The promoter activity could be measured for different situations or research questions, such as:
* estimation of the level of expression in comparison (relative) to some known value 
* how fast a gene is expressed after induction 
* the timing of expression relative to others genes 
* the specific spatial location of expression 
Methods to study promoter activity commonly are based in the expression of a reporter gene from the promoter of the gene of interest. Mutations and deletions are made in a promoter region, and their changes on couple expression of the reporter gene are measured.
The most important reporter genes  are the fluorescence proteins as GFP. These reporters allow to measure promoter activation by increasing fluorescent signals, and deactivation by decrease in the rate of fluorescence.