Apparatuses, methods, and systems are disclosed for sidelink HARQ operation. One apparatus includes a memory coupled to a processor, where the processor is configured to cause the second apparatus to receive a SL grant for a SL transmission using SL resources, the SL grant indicating a first HARQ process identified by a first HARQ process identifier, and to select a second SL HARQ process identified by a second SL HARQ process identifier for the SL transmission using the SL resources. The processor is configured to cause the second apparatus to transmit SCI containing the second SL HARQ process identifier and to perform the SL transmission using the SL resources.