Viridian Forest (Japanese: トキワの森Tokiwa Forest) is a forest which lies between Viridian City and Pewter City in the middle of Route 2 with both the north and south entrances of Viridian Forest open to the route. The deep and sprawling forest is said to be a natural maze as many people get lost inside. The trees in the forest are so thick that little light gets in, and because of this the forest is full of Bug- and Flying-typePokémon that attract Bug Catchers and other Trainers.
In Generation II, Viridian Forest no longer exists as a standalone area and has been replaced with a maze of standard trees, containing items and Berry trees, which is now a part of Route 2. The maze forms roughly the same design as before and the items are placed similarly to those located in Generations I and III. However, in the Generation IV remakes, the forest is re-expanded into the state of an actual dungeon, with five Trainers added, and with Pokémon similar to those of Pokémon Yellow.
Preview of Viridian Forest in FireRed and LeafGreen
Viridian Forest is a deep and sprawling woodland expanding across Route 2 reaching to Pewter City in the north and to Viridian City in the south, giving the forest its name. The trees in this area have grown so dense that light has trouble getting through, and because of this the forest seems dark and gloomy. The path across to each terminus winds its way around the dense layers of trees, creating a giant labyrinth many liken to a natural maze, as many people get lost inside. The entrance to the forest seems to be light and airy: the trees are not as thick and sunlight is able to get in fine. The trees stand tall and the grass appears to be long and unmaintained, while trees in the distance appear to be shadowed.
Because of its dark and shady environment, Viridian Forest hosts a large array of different Bug-type Pokémon. They are not particularly powerful, but they will cause status conditions such as paralysis and poison. It is advised from sign posts and even from other Trainers that Trainers should buy status condition healing items such as Antidote and Parlyz Heal before entering the forest. Pokémon such as Caterpie, Weedle, Metapod, and Kakuna inhabit trees and tall grass, allowing themselves to stay hidden from predators. Viridian Forest is also one of the locations where Trainers can seek wild Pikachu as well as under-leveled Pidgeotto, making the forest a very exclusive place to find Pokémon. Bug Catchers, who own common Bug-type Pokémon such as Caterpie and Weedle, come here in groups from Viridian and Pewter to battle and train.
The forest was also the setting of the following episode. While traveling through the forest, Ash spotted a Weedle and decided to catch it. He used Pidgeotto against it, but another Trainer named Samurai arrived on scene and challenged Ash to a battle. Meanwhile, the Weedle escaped and Samurai rebuked Ash for it. The two then engaged themselves in a Pokémon battle, which got stuck for hours as their Metapod were unable to do anything but use Harden. All of sudden, a swarm of Beedrill arrived and carried Ash's Metapod away. Samurai took Ash, Misty, and their other Pokémon to a safe place, but not before running into group of Kakuna which promptly evolved and attacked them. The next day, Ash went and saved his Metapod from under the very same tree, having noticed him there the day before. Due Team Rocket's loud entrance, the Beedrill woke up and started chasing Ash. He apologized to Metapod for leaving him in trouble like that, after which he saved his Trainer from a Beedrill's Twineedle attack. Afterwards, he quickly evolved into a Butterfree and saved Ash, Misty, and the samurai by putting the Beedrill asleep with Sleep Powder. At the end of the episode, Ash and Misty left Viridian Forest.
Later in the original series, Ash released his newly evolved Pidgeot near Viridian Forest in Pallet Party Panic. As the group went through Viridian Forest, Ash and his friends were attacked by a flock of Spearow. The flock's leader was a Fearow, which apparently evolved from the Spearow that Ash hit with a rock on the first day of his journey. Ash climbed a tree to escape and found Team Rocket with a flock of Pidgey and Pidgeotto who were in a turf war with the flock of Spearow. After sending Team Rocket falling from tree as a bait for the Spearow, Ash sent out his own Pidgeotto to tell the other bird Pokémon that it was safe to leave. The Fearow then came back, however, and started attacking bird Pokémon. Ash's Pidgeotto tried to fight the Fearow, but was badly hurt. Instead of giving up, however, it then evolved into a Pidgeot. Pidgeot and Pikachu then teamed up to defeat the Fearow and its minions. Ash also tried to catch the Fearow but failed, forcing them just to chase it away. Ash decided to leave Pidgeot in the forest to look after the Pidgey and Pidgeotto.
Many important people were born in or near the forest, including Lance, Giovanni, and Yellow. It is said that once every ten years, a child is born possessing the mysterious powers of the forest. Lance and Yellow both share the power to heal Pokémon using physical touch as well as the ability to read the minds of Pokémon. In addition, Yellow can also move inanimate objects, namely Poké Balls, as well as raise her Pokémon's levels using the power of Viridian Forest (although this power seems involuntary and temporary). Giovanni has yet to display any special powers.
The forest is first visited in The Secret of Kangaskhan by Red and Blue. In their search for Pokémon, they encounter a wildKangaskhan and its baby. Upon Red's return to Viridian City via the forest, he saves a young Yellow from being attacked by a wild Dratini.
The forest was the first place Yellow visited after she had assumed guardianship of Pika. She met Bill inside the forest, and not long after, both were attacked by Lorelei. However, Green helped them escape. Much later, Green also revealed that she was the one who set Yellow off on her journey after meeting her at Viridian Forest, recognizing her as not only having met Red who had disappeared, but also as the one that fit the criteria given to her by Silver. After teaching her the basics of battling, Green gave a straw hat to Yellow, telling her to disguise herself as a boy and not to reveal her name.
In Yellow, the sole Lass found here has her sprite based over the grass, instead of among it. She can't move because she doesn't have a range at which she will walk up to the player, so she can only be battled by talking to her. 
Despite the forest returning as a standalone area in HeartGold and SoulSilver, the background music is a remix of Viridian Forest (Japanese: トキワのもり) from Generation II, used on Route 2, with Route 2 in HeartGold and SoulSilver given Route 3 (Japanese: ３ばんどうろ) as the background music, which is also used for most other routes in Kanto on both Generation II and HeartGold and SoulSilver. Incidentally, Generation II's version of Viridian Forest, used on Route 2, was in itself a remix of Viridian Forest (Japanese: トキワの森) from Generation I.