Zewail's key work was a pioneer of femtochemistry—i.e. the study of chemical reactions across femtoseconds. Using a rapid ultrafast laser technique (consisting of ultrashort laser flashes), the technique allows the description of reactions on very short time scales – short enough to analyse transition states in selected chemical reactions. Zewail became known as the "father of femtochemistry". He also made critical contributions in ultrafast electron diffraction, which uses short electron pulses rather than light pulses to study chemical reaction dynamics.