Threshold is a space-themed fixed shooter written by Warren Schwader and Ken Williams for the Apple II and published by On-Line Systems in 1981.

The game was inspired by Ken Williams playing an Astro Blaster arcade machine in a store and calling Warren Schwader.

In an interview in Halcyon Days, Schwader said: Instead of trying to duplicate the game in every detail, I set out to just take the concept and run with it–after first playing Astro Blaster for hours on end though.

[5] Reviewing the Apple II original for Creative Computing, David Lubar wrote, "The animation in Threshold is superb", and he found the number of enemy types and waves to be a strong point.

"[8] Appraising the Atari 8-bit computer version, Electronic Games noticed the Astro Blaster connection and wrote "The graphics in Threshold are tremendous.