Unlike other industries, there is no generally accepted definition for what constitutes power in PR. Some individuals are powerful because of the profile of their company or clients. Others yield great influence in media or political circles. And there are those who are powerful because they have mentored leaders who carry on their philosophy. As communicators continue to assert a larger profile in companies and campaigns, PRWeek's editorial team looks across a range of criteria and individuals to devise this ranking of 25 industry leaders who wield power in ways that are critical for the profession's growth and development.