We analyzed in detail the electric field variations preceding the first return strokes of 80 cloud-to-ground lightning flashes in nine different storms observed at the NASA Kennedy Space Center during the summers of 1976 and 1977. The electric field variations are best characterized as having two sections: preliminary variations and stepped leader. The stepped-leader electric-field change begins during a transition period of a few milliseconds duration marked by characteristic bipolar pulses. The durations of stepped leaders lie most frequently in the range 6--20 milliseconds. We infer from our measurements and critical review of the previous literature that there is only one type of stepped leader, not the two types, α and β, often referred to in the literature.