To gain a rank in the U.S. Marines as of 2015, perform your duty adequately and meet the time in service and time in grade requirements. Promotions from E-1 to E-2 and E-2 to E-3 occur when the time requirements are met. Subsequent promotions have time requirements and can only occur when there is a vacancy in the higher rank.
A promotion from E-1 to E-2 requires six months time in service and six months time in grade, while a promotion from E-2 to E-3 requires nine months time in service and eight months time in grade. Since promotions to E-4 and higher require a vacancy in that higher rank, they are competitive. Promotions to E-4 and E-5 use each Marine's composite score. Several factors determine the composite score, including the Marine's average proficiency, conduct and assignment bonuses.
A promotion board that meets annually handles promotions to E-6, E-7, E-8 and E-9. Each rank has its own time requirements. The ranks also require certain professional military education courses.
A commander can promote Marines up to E-8 using the meritorious promotion system. This promotion system is for outstanding Marines, and the commander can give the promotion for combat or non-combat reasons. Minimum time in service requirements still apply for these promotions.