Callum James Henderson-Begg (born 17 January 1981), known as Cal Henderson, is a British computer programmer and author based in San Francisco.

Henderson attended Sharnbrook Upper School and Community College,[1] and Birmingham City University where he graduated with a degree in software engineering in 2002.

[2] Henderson is best known as the co-founder and chief technology officer at Slack, as well as co-owning and developing the online creative community B3ta[3] with Denise Wilton and Rob Manuel; being the chief software architect for the photo-sharing application Flickr[4] (originally working for Ludicorp[5][6] and then Yahoo); and writing the book Building Scalable Web Sites[7] for O'Reilly Media.

He has also worked for EMAP as their technical director of special web projects[8] and is responsible for writing City Creator[9] among many other websites, services and desktop applications.

[4] He is also a frequent contributor to open-source software projects and runs a number of utility websites, such as Unicodey, to make certain programming tasks easier.