Beginner
Documentation (flask.pocoo.org)
Flask Mega Tutorial (blog.miguelgrinberg.com)
Explore Flask (exploreflask.com)
What is a Web Framework? (jeffknupp.com)
Flask Installation (flask.pocoo.org)
Flask Tutorial (learn.rmotr.com)
Flask SQLAlchemy (www.youtube.com)
Intermediate
Flaskr: Intro to Flask and Test Driven Development (github.com)
Discover Flask (discoverflask.com)
Advanced