In the past decade, earthquake scientists have discovered a family of unusually slow earthquakes. These slow earthquakes occur in diverse geologic environments. Like ordinary earthquakes, they occur as slip on the same faults that host ordinary earthquakes, but they take a long time to unfold, such that they can be described as "slow." Their discovery was enabled by deployment of highly sensitive eart tworks. Slow earthquakes are slow in a systematic way that leads us to define them as a new earthquake category in much the same way that astronomers categorized main-sequence and off-main-sequence stars nearly a century ago. Unlike ordinary earthquakes, which grow explosively in size with increasing duration, slow earthquakes, whether large or small, grow at a constant rate. They occur on the deep extension of large faults, a location that is "strategic" because it adjoins the part of the faults that generate the more familiar, and dangerous, "ordinary" earthquakes. Slow earthquakes have the potential to trigger large earthquakes. For this reason alone they merit intense study. Their recent discovery also points out that there is much still have to learn about earthquakes, and that earthquake science is a field where fundamental discoveries can be expected.