Evolution

The Basics Of Evolution
By now you've probably noticed that as your Pokémon fights, it gains Experience (EXP). As it gains EXP, it's level rises. When it's level rises to a certain point, it will change it's form, to a different, more powerful creature, and you will recieve credit for catching another Pokémon. Some Pokémon evolve once, while others can evolve twice. Some, like Tyrogue, Eevee, Slowpoke, and others, have multiple paths of evolution. There are four main ways that your Pokémon can evolve- through gaining EXP, through stones, though trading the Pokémon with another game, and Happiness. While evolution is usually a good idea, there are some things that should be taken into consideration, which will be outlined below.

Experience Evolution
Experience Evolution is simply evolution that occurs when your Pokémon reaches a certain level. Pidgey evolves into Pidgeotto at Lv. 18, for example. Most Pokémon evolve this way. If you don't want your Pokémon to evolve, just press the B Button as it's evolving and it will wait a level to evolve again.

Stone Evolution
Unlike Experience Evolution, Stone Evolution can happen anytime- as long as you have the proper stone. Exeggcute evolves into Exeggutor when you use the Leaf Stone, and it doesn't matter what the level is. Keep in mind that Stone Evolution cannot be stopped, so don't use the Stone until you're sure. There are six different stones.

Fire Stone:
Eevee evolves into Flareon
Vulpix evolves into Ninetails
Growlithe evolves into Arcanine
Water Stone:
Eevee evolves into Vaporeon
Shellder evolves into Cloyster
Poliwhirl evolves into Poliwrath
Staryu evolves into Starmie
Thunder Stone:
Eevee evolves into Jolteon
Pikachu evolves into Raichu
Leaf Stone:
Exeggcute evolves into Exeggutor
Gloom evolves into Vileplume
Weepinbell evolves into Victreebel
Moon Stone
Clefairy evolves into Clefable
Jigglypuff evolves into Wigglytuff
Nidorino evolves into Nidoking
Nidorina evolves into Nidoqueen
Sun Stone
Gloom evolves into Bellossom
Sunkern evolves into Sunflora

Trade Evolution
Like Stone Evolution, Trade Evolution can take place at any level. Graveler evolves into Golem when it is traded from one Game Boy to another. Note that you cannot stop trade evolution from occuring. If you don't want your Pokémon to evolve, then don't trade it! If a Pokémon requires an Item to evolve, the Item will be lost.

Trade:
Machoke evolves into Machamp
Graveler evolves into Golem
Kadabra evolves into Alakazam
Haunter evolves into Gengar
Porygon evolves into Porygon2 with the Up-Grade from Silph Co.
Onix evolves into Steelix with the Metal Coat from the SS Aqua
Scyther evolves into Scizor with the Metal Coat as well
Poliwhirl evolves into Politoed with the King's Rock from the Slowpoke Well
Slowpoke evolves into Slowking with the King's Rock too
Seadra evolves into Kingdra with the Dragon Skin from the cave between Ecruteak and Mahogany

Happiness Evolution
One of the more irritating ways to evolve a Pokémon is though Happiness. When they get REALLY happy, they'll evolve. The quickest way to do this is to give them lots of items like Protein and HP Up. Catch them with Love Balls, don't give them bad items and DO NOT trade them or put them on Stadium's boxes.

Happiness:
Togepi evolves into Togetic
Pichu evolves into Pikachu
Cleffa evolves into Clefairy
Igglybuff evolves into Jigglypuff
Chansey evolves into Blissey
Golbat evolves into Crobat
Eevee evolves into Espeon during the day and Umbreon during the night

Effects of Evolving Pokémon

Status Increase
When a Pokémon evolves, it's stats rise. Simple as that. The Base Stat values increase, and the stats enjoy a sizeable boost. See what happens when Dragonair evolves into Dragonite:

Lv. 55	Dragonair	Dragonite
Health	183	216
Attack	148	203
Defence	128	161
Speed	133	144
S. Attack	133	166
S. Defence	133	166

In one action, the Lv. 55 Dragonair evolved into the Lv. 55 Dragonite, greatly increasing all of it's stats. Take note, however, that there are some Pokémon who have a stat that actually goes down: Poliwhirl has better Speed then Poliwrath; Butterfree's Defence isn't as high as Metapod's. However, the one falling stat is more then made up for by the rise in all the other stats, and this should not be enough of a concern to prevent evolution completely.

Attack Changes
(All examples here are RBY.)
Each of the four different kinds of Pokémon react to evolution in different ways. Experience Evolution will cause Pokémon to learn attacks later then their preevolved forms. Squirtle learns Hydro Pump at Lv. 42, while it's fully evolved form Blastoise learns the move at Lv. 52. There are only two instances where this isn't so: Charmeleon learns Fire Spin at Lv. 56, whereas Charizard learns it at Lv. 55. Omastar also learns all of it's attacks before it's preevolved form Omanyte.

Stone Evolution Pokémon, on the other hand, suffer a much more severe penalty for evolution: They cannot learn ANY new moves by themselves. If you evolve a Pikachu right away, no matter what it's level is, it won't learn any of it's natural attacks. There are a few rare cases where a Pokémon may learn one extra move that it couldn't learn otherwise; Cloyster learns Spike Cannon, Nidoking learns Thrash. Most of the time, however, these secret techniques aren't worth losing the rest of a Pokémon's abilities. The solution? Wait until the Pokémon has learned all the attacks it needs, then evolve it. The one big exception to this rule, of course, is Eevee. Evolve it, or it won't learn ANY of it's elemental attacks. (You can wait for Focus Energy or Baton Pass, though.)

Now, we come to the Trade Evolution Pokémon, who seem to get off scot-free. Trade Evolution Pokémon learn all of their attacks at exactly the same time, whether they're evolved or not. Haunter learns Dream Eater at Lv. 38, and so does Gengar. In the case of Trade Evolution Pokémon, you should evolve them as soon as you get the chance.

Happiness Pokémon usually don't notice a difference in their attacks, like Trade Evolution. However, Blissey learns its attacks earlier, and the babies- Pichu, Cleffa and Igglybuff- have their own movesets. Check the Gold and Silver Profiles for info.

Evolution Quirks
Regarding Experience Evolution: It may seem like a good idea to hold back your Pokémon's evolution until it learns all it's attacks. And it is, in most cases. However, some Pokémon learn a move when they evolve that they wouldn't otherwise be able to obtain. When Abra evolves into Kadabra at Lv. 16, it learns Confusion. If for some reason you did not let it evolve until, say, Lv. 19, it would never be able to learn Confusion.

Another thing to keep in mind is that some Pokémon don't learn ANY attacks until they evolve! Most cannot use TM's either, making evolution the only way to go.
Caterpie and Weedle will learn Harden when they evolve into Metapod and Kakuna, but that's it until they evolve into Butterfree and Beedrill.
Magikarp must evolve into Gyarados before its abilities can be learned.
Abra will miss all of it's techniques unless it evolves into Kadabra. Note that Abra can still use TM's.
Phanpy and Donphan, Smoochum and Jynx, and Sunkern and Sunflora have wildly varying movesets. Again, check the Profiles to decide when they should evolve.