HeartGold & SoulSilver Evolution Chart
Each HeartGold and SoulSilver evolution with its method, plus the four Gen 4 evolutions that only work in Sinnoh.
ALL HEARTGOLD & SOULSILVER EVOLUTIONS
HGSS brings Gen 4's cross-generation evolutions to Johto. Tangrowth, Yanmega and Mamoswine all evolve by leveling up while knowing AncientPower. The place-based ones didn't make the trip: with no Moss Rock, Ice Rock or Mt. Coronet here, Leafeon, Glaceon, Magnezone and Probopass have to come from Diamond, Pearl or Platinum.
HOW TO EVOLVE POKEMON
Most of them, and several wait until after the League. Tangela and Yanma learn AncientPower at level 33, which evolves them into Tangrowth and Yanmega; Piloswine only has it as a starting move, so breed or tutor it for Mamoswine. Weavile needs a Razor Claw at night, and Honchkrow and Mismagius need Dusk Stones. Leafeon, Glaceon, Magnezone and Probopass can't evolve in HGSS at all.
You can't in HeartGold or SoulSilver. Leafeon needs Eevee leveled up in Eterna Forest, home of the Moss Rock, and Glaceon needs Route 217 and its Ice Rock. Both places are only in Diamond, Pearl and Platinum, so evolve Eevee there and trade it over.
The Pokewalker (pedometer accessory) earns experience and can increase friendship for the Pokemon stored inside. Walking enough steps boosts friendship, which can help trigger friendship evolutions faster. You can also find evolution items on certain Pokewalker routes, like stones from specific course rewards.
Bill's grandfather on Route 25 gives you stones for showing him specific Pokemon. Bug-Catching Contest prizes include Sun Stones. The Pokeathlon Dome shop sells all stones for Athlete Points on specific days of the week. That shop is the most reliable repeatable source once you have enough points.
Technically yes, but it's slow. Friendship builds from walking, leveling, and grooming at the Goldenrod haircut shop. Before Goldenrod, your options are limited to just walking and fighting. Crobat is one of the most popular early friendship evolutions since Zubat is everywhere in caves from the start.