One of the most popular methods for complete characterization of ultra-short laser pulses is the Frequency-Resolved Optical Gating (FROG). In FROG, the acquired data is the power spectrum of the product of the unknown pulse with its delayed replica. Therefore the measured signal, called FROG trace, is a quartic function of the unknown pulse. In this talk, I will describe the FROG method, discuss its symmetries and present the requirement for unique mapping between a pulse and its FROG trace.