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Building Slack - Lessons Learned from Co Founder Cal Henderson

Over the last seven years, Slack has dominated the productivity and communications space. Initially starting off as a tool for startups and small teams, the company quickly scaled its capabilities to serve large, distributed teams.

Recently Cal Henderson, Co-Founder and CTO of Slack, sat down with Business Insider to share the original vision, category creation tips and give us a glimpse of what it was like in the first 100 days of shaping Slack. Here are the key takeaways:

Slack was born from failed attempts to build a video game

In the early 2000s, Cal and his founding team tried countlessly to build a video game. While the attempt to create the video game failed, out of that failure the crew created Flicker, which was later sold to Yahoo for approximately $25 million. In 2009, the team tried again to create another video game, spent four years and failed again. In 2013, the founders realized that the way they worked together was their competitive advantage. Over the course of developing the game, they developed a set of tools that allowed the founders to get work done. They realized that after 4 years, 50 distributed employees,  they developed tools for better communications. Out of that came the idea to develop Slack.

Originating as a communications platform, Slack evolved into a channel-based messaging tool that brought together all of the other work-related software products.

In the beginning, Slack was a tough sell

Convincing people was hard because Slack created a new category and admittedly did not have a crystalized message. It took a lot of repetition, convincing and burning social capital.

Audio, a US-based Spotify competitor, was the first company to try the platform outside of Slack’s immediate team. The team at Slack learned a lot in the first couple of days around customer needs, product design and UI. That feedback loop helped to polish the product and moved it to the next phase.

Since the Slack team pivoted from using email to using Slack for internal communication very early on, the group was able to spend time finding bugs, iterating, and building a better version of the tool. That was an accelerant  a big advantage early on for Slack.

How you allocate time as a leader matters

When slack was 8 people, every hour of every day counted to get to the next milestone. The founding team was all in the weeds writing code, fixing bugs, etc. Today with over 2,000 employees all over the world, the day to day for Cal changed significantly. No more writing or publishing code, instead the focus shifted to leading global engineering teams for the next stage of Slack's growth. The goal centered around understanding how to build an easily replicated model and to step back from hands-on role and to allow other people to do this all in 6-12 months.


“Its very easy to be busy all the time and its much harder to make sure you are spending the amount of time on long-term vision and success of your organization.” Cal Henderson, Co-founder and CTO of Slack


Look at how you spend your time - always allocate time to focus on business strategy is important so as not to fall in the well of reactiveness. Figure out what you should be doing 6 months from now versus iterating on the known problem set. Paint the vision of the future and figure out what needs to be done to get there. Aligning global teams requires a crisp vision of the future and the discipline to paint that picture for your teams. Drive people towards the future vision - not a set of tasks.  

Product, business model and culture

According to Cal product, business and culture are three components that companies are always building.  Slack chose to be intentional about the culture so 5-7 years down the line people would want to continue working at the company.

“ People you work with the closest and  your relationship with them,  is the biggest thing that will influence experience at building a company.” Cal Henderson, Co-founder and CTO of Slack


You spend so much time with your co-workers that you have to like spending time with them. What's important is to enjoy spending time with colleagues even when things are going poorly not just when things are going well. Being able to work through that constructively and come out on the other side is extremely important. It's no doubt that Slack is here to stay. Even with recent pandemic, the need has strengthened for easy communication across teams. It's great to hear that as a founder Cal remains focused on building an amazing product and creating a great culture for his global teams.