In the past decades, many failure incidents for transmission line structures were observed during High Intensity Wind (HIW) events, in the form of downbursts and tornados, in North America, Australia, and other locations around the globe. Examining design codes pertaining to this type of structures reveals the lack of procedures to determine the wind loading acting on transmission tower systems due to High Intensity events. A major challenge in the analysis and design of structures under HIW is the localized nature of these events, which makes the forces acting on the towers and lines dependent on the location and characteristics of the event. Motivated by the failure of number of transmission towers in Canada, an extensive research program was initiated at the University of Western Ontario (UWO) a decade ago and is still progressing with final aim for developing knowledge and information for designing transmission line structures to sustain HIW events. The current paper covers the two types of HIW events: tornados and downbursts. For each event, a literature review is provided followed by a summary of the outcomes of the research conducted at UWO and a description of the wind field. The main contribution in this paper is the introduction of procedures to account for the critical effects of HIW on transmission line structures. Using the knowledge gained from years of research on this subject, critical load cases and load profiles simulating the downburst and tornado configurations that are critical for transmission towers are identified and presented in a format that can be implemented in design codes and can be used by practitioners.