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Second-Harmonic Generation Frequency-Resolved Optical Gating (SHG-FROG) is a powerful and widely used technique for the complete characterization of ultrashort laser pulses. While simple autocorrelators can only estimate pulse duration, SHG-FROG provides a full measurement of the pulse intensity and phase in time. The core principle of FROG is to create a spectrally resolved autocorrelation. In the specific case of SHG-FROG, this is achieved by splitting the pulse into two replicas and introducing a variable time delay between them. These two pulses are then focused into a non-linear crystal where they generate a second-harmonic signal, but only when they overlap in time. This generated signal intensity is recorded by a spectrometer as a function of the delay τ. The resulting dataset is a two-dimensional plot known as a FROG trace, where one axis is the time delay, and the other is the frequency. This trace acts as a unique "fingerprint" of the pulse. The true power of SHG-FROG lies in its retrieval algorithm. Using an iterative algorithm, the measured FROG trace is processed to uniquely retrieve the electric field of the original pulse, revealing both its intensity profile and its phase profile.