This depends on the generation of Pokemon game you are playing. For example, in Generation I, Pikachu can only learn Flash. Below is a list of Pokemon Generations and the HMs that Pikachu can learn: Generation I: Flash (HM 05) Generation II: Strength (HM 04), Flash (HM 05) Generation III: Strength (HM 04), Flash (HM 05), Rock Smash (HM 06) Generation IV: Strength (HM 04), Rock Smash (HM 06) Generation V: Strength (HM 04) Generation VI: Strength (HM 04)