From their website:
Our mission is to advance designing as a professional craft, strategic tool, and vital cultural force.
Founded in 1914, AIGA is the oldest and largest membership association for design professionals. Today the national organization represents more than 20,000 designers participating in 65 local chapters and more than 200 student groups.
AIGA supports design professionals, educators, and students at whatever stage they are in their careers. The association spans the many varied branches of the profession: from book design, typography, and other types of traditional communication design to newer disciplines such as interaction design and experience design.
AIGA has become a forum for design professionals to exchange ideas and information, participate in critical analysis and research, and advance education and ethical practice.
When you become a member of AIGA Seattle, you join forces with nearly 700 designers across the state of Washington and 20,000 designers nationwide. AIGA Seattle unites designers from all disciplines and areas, from the tech-focused Puget Sound area to the Design for Democracy campaign in Olympia.
Joining as professional designer will cost you $295 per year in membership dues, joining now as a student gives you access to luncheons, events, and resources for $75 per year.
More information can be found at Seattle AIGA.