Slack's process was one of the most in-depth I've experienced. They're extremely thorough in their review of your work history and ensuring that there's a cultural fit. Overall I had more than 10 scheduled interviews plus ongoing phone calls with the internal recruiter. Each conversation built on the last, and there was a clear path to the final decision. They adopted a behavioral interview style, asking for examples of particular capabilities and attributes. With each meeting, I could see in the interviewer the enthusiasm, intelligence and humility that is so important to Slack's culture. While the questions were sometimes rigorous, the conversations were free-flowing and the team went out of their way to make me comfortable.