Ultrasonic waves have a long history in detection of surface breaking cracks. Attempts are being made to use guided waves as a defect detection tool in aging skin structures in aircrafts and in the power generation industries as these waves offer a great advantage over conventional bulks waves. Guided waves can be excited at one position and allowed to propagate considerable distances before attenuating. Depending on the configuration employed for defect detection, reflected or received waveforms give information regarding the integrity of the structure along the line of sight. This description makes the technique look rather simple. Particularly, NDT utilizing Lamb waves is more complex due to the existence of two or more modes at any given frequency. Success was reported by several authors on defect detection using Lamb waves. Brief or no explanation was given on the reasons behind the choice of specific excitation frequencies and incident angles. The emphasis was solely on the defect detection aspects.