Flash is learned by 5 species by level and 299 by machine

Effect, in PokéAPI's words

“Lowers the target’s accuracy by one stage.”

Who learns it

301 species in all, 5 of 22 games shown

Brilliant Diamond and Shining Pearl: 208 species

Species	Method	Level
Bulbasaur	Machine
Ivysaur	Machine
Venusaur	Machine
Butterfree	Machine
Beedrill	Machine
Pikachu	Machine
Raichu	Machine
Clefairy	Machine
Clefable	Machine
Jigglypuff	Machine
Wigglytuff	Machine
Oddish	Machine
Gloom	Machine
Vileplume	Machine
Paras	Machine
Parasect	Machine
Venonat	Machine
Venomoth	Machine
Meowth	Machine
Persian	Machine
Psyduck	Machine
Golduck	Machine
Abra	Machine
Kadabra	Machine
Alakazam	Machine
Bellsprout	Machine
Weepinbell	Machine
Victreebel	Machine
Slowpoke	Machine
Slowbro	Machine
Magnemite	Machine
Magneton	Machine
Drowzee	Machine
Hypno	Machine
Voltorb	Machine
Electrode	Machine
Exeggcute	Machine
Exeggutor	Machine
Koffing	Machine
Weezing	Machine
Chansey	Machine
Tangela	Machine
Staryu	Machine
Starmie	Machine
Mr. Mime	Machine
Jynx	Machine
Electabuzz	Machine
Jolteon	Machine
Porygon	Machine
Zapdos	Machine
Mewtwo	Machine
Mew	Machine
Chikorita	Machine
Bayleef	Machine
Meganium	Machine
Ledyba	Machine
Ledian	Machine
Spinarak	Machine
Ariados	Machine
Chinchou	Machine
Lanturn	Machine
Pichu	Machine
Cleffa	Machine
Igglybuff	Machine
Togepi	Machine
Togetic	Machine
Natu	Machine
Xatu	Machine
Mareep	Machine
Flaaffy	Machine
Ampharos	Machine
Bellossom	Machine
Hoppip	Machine
Skiploom	Machine
Jumpluff	Machine
Sunkern	Machine
Sunflora	Machine
Yanma	Machine
Wooper	Machine
Quagsire	Machine
Espeon	Machine
Umbreon	Machine
Slowking	Machine
Misdreavus	Machine
Girafarig	Machine
Shuckle	Machine
Skarmory	Machine
Porygon2	Machine
Stantler	Machine
Smoochum	Machine
Elekid	Machine
Blissey	Machine
Raikou	Machine
Entei	Machine
Lugia	Machine
Ho-Oh	Machine
Celebi	Machine
Treecko	Machine
Grovyle	Machine
Sceptile	Machine
Beautifly	Machine
Dustox	Machine
Lotad	Machine
Lombre	Machine
Ludicolo	Machine
Seedot	Machine
Nuzleaf	Machine
Shiftry	Machine
Ralts	Machine
Kirlia	Machine
Gardevoir	Machine
Surskit	Machine
Masquerain	Machine
Shroomish	Machine
Breloom	Machine
Nincada	Machine
Ninjask	Machine
Shedinja	Machine
Skitty	Machine
Delcatty	Machine
Sableye	Machine
Meditite	Machine
Medicham	Machine
Electrike	Machine
Manectric	Machine
Plusle	Machine
Minun	Machine
Volbeat	Machine
Illumise	Machine
Roselia	Machine
Spoink	Machine
Grumpig	Machine
Spinda	Machine
Cacnea	Machine
Cacturne	Machine
Lunatone	Machine
Solrock	Machine
Baltoy	Machine
Claydol	Machine
Lileep	Machine
Cradily	Machine
Castform	Machine
Kecleon	Machine
Shuppet	Machine
Banette	Machine
Duskull	Machine
Dusclops	Machine
Tropius	Machine
Chimecho	Machine
Absol	Machine
Snorunt	Machine
Glalie	Machine
Metang	Machine
Metagross	Machine
Latias	Machine
Latios	Machine
Jirachi	Machine
Turtwig	Machine
Grotle	Machine
Torterra	Machine
Kricketune	Machine
Shinx	Machine
Luxio	Machine
Luxray	Machine
Budew	Machine
Roserade	Machine
Wormadam	Machine
Mothim	Machine
Vespiquen	Machine
Pachirisu	Machine
Cherubi	Machine
Cherrim	Machine
Gastrodon	Machine
Drifloon	Machine
Drifblim	Machine
Mismagius	Machine
Glameow	Machine
Purugly	Machine
Chingling	Machine
Bronzor	Machine
Bronzong	Machine
Mime Jr.	Machine
Happiny	Machine
Spiritomb	Machine
Skorupi	Machine
Drapion	Machine
Carnivine	Machine
Finneon	Machine
Lumineon	Machine
Snover	Machine
Abomasnow	Machine
Magnezone	Machine
Tangrowth	Machine
Electivire	Machine
Togekiss	Machine
Yanmega	Machine
Leafeon	Machine
Porygon-Z	Machine
Gallade	Machine
Dusknoir	Machine
Froslass	Machine
Rotom	Machine
Uxie	Machine
Mesprit	Machine
Azelf	Machine
Dialga	Machine
Cresselia	Machine
Manaphy	Machine

Let's Go Pikachu and Let's Go Eevee: 2 species

Species	Method	Level
Kadabra	Level up	1, 13
Alakazam	Level up	1, 13

Ultra Sun and Ultra Moon: 3 species

Species	Method	Level
Volbeat	Level up	1
Shiinotic	Level up	1, 8
Morelull	Level up	8

Sun and Moon: 3 species

Species	Method	Level
Volbeat	Level up	1
Shiinotic	Level up	1, 8
Morelull	Level up	8

Omega Ruby and Alpha Sapphire: 297 species

Species	Method	Level
Volbeat	Level up, Machine	1
Bulbasaur	Machine
Ivysaur	Machine
Venusaur	Machine
Butterfree	Machine
Beedrill	Machine
Pikachu	Machine
Raichu	Machine
Clefairy	Machine
Clefable	Machine
Jigglypuff	Machine
Wigglytuff	Machine
Oddish	Machine
Gloom	Machine
Vileplume	Machine
Paras	Machine
Parasect	Machine
Venonat	Machine
Venomoth	Machine
Meowth	Machine
Persian	Machine
Psyduck	Machine
Golduck	Machine
Abra	Machine
Kadabra	Machine
Alakazam	Machine
Bellsprout	Machine
Weepinbell	Machine
Victreebel	Machine
Slowpoke	Machine
Slowbro	Machine
Magnemite	Machine
Magneton	Machine
Drowzee	Machine
Hypno	Machine
Voltorb	Machine
Electrode	Machine
Exeggcute	Machine
Exeggutor	Machine
Koffing	Machine
Weezing	Machine
Chansey	Machine
Tangela	Machine
Staryu	Machine
Starmie	Machine
Mr. Mime	Machine
Jynx	Machine
Electabuzz	Machine
Jolteon	Machine
Porygon	Machine
Zapdos	Machine
Mewtwo	Machine
Mew	Machine
Chikorita	Machine
Bayleef	Machine
Meganium	Machine
Ledyba	Machine
Ledian	Machine
Spinarak	Machine
Ariados	Machine
Chinchou	Machine
Lanturn	Machine
Pichu	Machine
Cleffa	Machine
Igglybuff	Machine
Togepi	Machine
Togetic	Machine
Natu	Machine
Xatu	Machine
Mareep	Machine
Flaaffy	Machine
Ampharos	Machine
Bellossom	Machine
Hoppip	Machine
Skiploom	Machine
Jumpluff	Machine
Sunkern	Machine
Sunflora	Machine
Yanma	Machine
Wooper	Machine
Quagsire	Machine
Espeon	Machine
Umbreon	Machine
Slowking	Machine
Misdreavus	Machine
Girafarig	Machine
Shuckle	Machine
Skarmory	Machine
Porygon2	Machine
Stantler	Machine
Smoochum	Machine
Elekid	Machine
Blissey	Machine
Raikou	Machine
Entei	Machine
Lugia	Machine
Ho-Oh	Machine
Celebi	Machine
Treecko	Machine
Grovyle	Machine
Sceptile	Machine
Beautifly	Machine
Dustox	Machine
Lotad	Machine
Lombre	Machine
Ludicolo	Machine
Seedot	Machine
Nuzleaf	Machine
Shiftry	Machine
Ralts	Machine
Kirlia	Machine
Gardevoir	Machine
Surskit	Machine
Masquerain	Machine
Shroomish	Machine
Breloom	Machine
Nincada	Machine
Ninjask	Machine
Shedinja	Machine
Skitty	Machine
Delcatty	Machine
Sableye	Machine
Meditite	Machine
Medicham	Machine
Electrike	Machine
Manectric	Machine
Plusle	Machine
Minun	Machine
Illumise	Machine
Roselia	Machine
Spoink	Machine
Grumpig	Machine
Spinda	Machine
Cacnea	Machine
Cacturne	Machine
Lunatone	Machine
Solrock	Machine
Baltoy	Machine
Claydol	Machine
Lileep	Machine
Cradily	Machine
Castform	Machine
Kecleon	Machine
Shuppet	Machine
Banette	Machine
Duskull	Machine
Dusclops	Machine
Tropius	Machine
Chimecho	Machine
Absol	Machine
Snorunt	Machine
Glalie	Machine
Metang	Machine
Metagross	Machine
Latias	Machine
Latios	Machine
Jirachi	Machine
Deoxys	Machine
Turtwig	Machine
Grotle	Machine
Torterra	Machine
Kricketune	Machine
Shinx	Machine
Luxio	Machine
Luxray	Machine
Budew	Machine
Roserade	Machine
Wormadam	Machine
Mothim	Machine
Vespiquen	Machine
Pachirisu	Machine
Cherubi	Machine
Cherrim	Machine
Gastrodon	Machine
Drifloon	Machine
Drifblim	Machine
Mismagius	Machine
Glameow	Machine
Purugly	Machine
Chingling	Machine
Bronzor	Machine
Bronzong	Machine
Mime Jr.	Machine
Happiny	Machine
Spiritomb	Machine
Skorupi	Machine
Drapion	Machine
Carnivine	Machine
Finneon	Machine
Lumineon	Machine
Snover	Machine
Abomasnow	Machine
Magnezone	Machine
Tangrowth	Machine
Electivire	Machine
Togekiss	Machine
Yanmega	Machine
Leafeon	Machine
Porygon-Z	Machine
Gallade	Machine
Dusknoir	Machine
Froslass	Machine
Rotom	Machine
Uxie	Machine
Mesprit	Machine
Azelf	Machine
Dialga	Machine
Cresselia	Machine
Manaphy	Machine
Darkrai	Machine
Shaymin	Machine
Arceus	Machine
Victini	Machine
Snivy	Machine
Servine	Machine
Serperior	Machine
Watchog	Machine
Pansage	Machine
Simisage	Machine
Munna	Machine
Musharna	Machine
Blitzle	Machine
Zebstrika	Machine
Woobat	Machine
Swoobat	Machine
Audino	Machine
Sewaddle	Machine
Swadloon	Machine
Leavanny	Machine
Cottonee	Machine
Whimsicott	Machine
Petilil	Machine
Lilligant	Machine
Sigilyph	Machine
Yamask	Machine
Cofagrigus	Machine
Gothita	Machine
Gothorita	Machine
Gothitelle	Machine
Solosis	Machine
Duosion	Machine
Reuniclus	Machine
Deerling	Machine
Sawsbuck	Machine
Emolga	Machine
Foongus	Machine
Amoonguss	Machine
Frillish	Machine
Jellicent	Machine
Joltik	Machine
Galvantula	Machine
Ferroseed	Machine
Ferrothorn	Machine
Eelektrik	Machine
Eelektross	Machine
Elgyem	Machine
Beheeyem	Machine
Litwick	Machine
Lampent	Machine
Chandelure	Machine
Stunfisk	Machine
Golett	Machine
Golurk	Machine
Virizion	Machine
Zekrom	Machine
Meloetta	Machine
Genesect	Machine
Chespin	Machine
Quilladin	Machine
Chesnaught	Machine
Vivillon	Machine
Flabébé	Machine
Floette	Machine
Florges	Machine
Furfrou	Machine
Espurr	Machine
Meowstic	Machine
Spritzee	Machine
Aromatisse	Machine
Swirlix	Machine
Slurpuff	Machine
Inkay	Machine
Malamar	Machine
Helioptile	Machine
Heliolisk	Machine
Amaura	Machine
Aurorus	Machine
Sylveon	Machine
Dedenne	Machine
Carbink	Machine
Pumpkaboo	Machine
Gourgeist	Machine
Bergmite	Machine
Avalugg	Machine
Xerneas	Machine
Diancie	Machine
Hoopa	Machine

Changed over time

One change since Generation I

1. Through Gen IIIAccuracy 70%100% from Diamond and Pearl, Generation IV