Pumpkin Seeds

Pumpkin Seeds are an item that can be used to grow Pumpkin plants.

Natural generation
Minecart with chests of abandoned mine shafts may contain pumpkin seeds in stacks of 2 to 4.

Pumpkin stems
Pumpkin stems, when destroyed, may drop 0-3 pumpkin seeds.

Nether reactor
In Pocket Edition, they are only obtainable through the Nether Reactor.

Farming
Pumpkin seeds can be planted only on farmland. Over time, they grow into a stem and produce a pumpkin on any adjacent Dirt, Grass Block or Farmland block. If a pumpkin is already occupying an adjacent dirt/grass or farmland spot, the stem will not grow more pumpkins until the pumpkin is removed. A single stem can grow an unlimited number of pumpkins. Like other seeds, they need a certain amount of light to grow. Pumpkin seeds are affected by bone meal only with respect to stem growth -- bone meal does not help produce the actual melons.