Apparatuses, methods, and systems are disclosed for power headroom report generation. One method includes aggregating multiple serving cells. The method includes determining that a power headroom report is triggered. The method includes receiving, at a first time after the power headroom report is triggered, a first uplink grant that allocates resources for a new transmission on a serving cell of the multiple serving cells. The method includes determining a power headroom value for each serving cell of the multiple serving cells being activated based on information received prior to and including the first time. The method includes generating a power headroom report medium access control control element including at least the power headroom value for each serving cell of the multiple serving cells being activated.