Lamb waves are currently used in many nondestructive testing applications to determine or monitor elastic properties of plate-like structures using various excitation methods. The methods vary from point-like dry contact transducers and laser ultrasound techniques in hostile environments to wedge transducers for inspection of immersed structures. In this paper, we analyze the excitation of Lamb waves in 2-D geometry by surface sources, as in the case of many applications. To calculate the coupling characteristics to various Lamb waves, we use the mode orthogonality principle and utilize the angular spectrum approach to find the field distributions on the surface excited by non contact transducers with finite dimensions. We present theoretical results related to Hertzian contact transducers at low frequencies along with some measurements in agreement with theory. We also investigate the wedge transducers exciting different Lamb wave modes in immersion mode.