Clinical Question: In adults with cervicogenic headache (CGH), what are the effects of a clinician-applied Mulligan Concept C1–C2 rotational sustained natural apophyseal glide on cervicogenic symptomology? Clinical Bottom Line: Both Level 2b and Level 4 evidence of Grade B quality exists to support using the C1–C2 rotational sustained natural apophyseal glide to decrease CGH severity and participant-reported neck disability immediately posttreatment, while also increasing cervical range of motion in adult patients with CGH headaches and a positive flexion–rotation test. Further research should examine the long-term effects of the C1–C2 rotational sustained natural apophyseal glide technique on cervical range of motion, flexion–rotation test results, and CGH frequency and severity. In addition, the flexion–rotation test should be explored as a standard diagnostic assessment in CGH cases.