Empire Earth/Resources

Most resource gathering methods can be upgraded in Capitols/Town Centers, causing Citizens to gather that particular resource faster. Citizens can carry a maximum of 15 units and drop them off at the nearest Settlement, Town Center or Capitols.

Food
Gained from:
 * Harvesting
 * Hunting
 * Fishing (by fishing ships, Docks serve as drop-off point)
 * Farming

Food is in high demand throughout all epochs as all infantry, tanks and Cybers require it for production. The speed of Harvesting and Hunting can be upgraded at Capitols/Town Centers while Farming is upgraded at the Granary. Fishing speed appears to be upgraded in parallel with fishing ships.

All land-based sources of Food can be dropped off at the Granary as well.

Once sources of Harvesting and Hunting disappear (random maps have regenerating herds of animals but they won't respawn if the entire herd is killed), Fishing will become the main source of food until Granary upgrades make Farming faster. By that point, fish will become scarce and enemy warships will prevent fishing boats from doing their job.

Wood
Gained from:
 * Trees

Wood is usually plentiful in supply as each tree contains about 500 units. The speed of gathering can be upgraded at Capitols/Town Centers. All ships, archers and siege weapons as well as most buildings require wood in the hundreds, therefore it must be gathered constantly.

Stone
Gained from:
 * Stone mines

Stone is a secondary building resource as some more advanced buildings (Universities, Hospitals and Forts) as well as all walls, towers and gates require it. Unlike wood, stone doesn't need to be gathered constantly as you rarely need it. Only six Citizens can gather from one stone mine but that much is usually enough unless you extensively rely on static defenses or try to build multiple Wonders.

Gold
Gained from:
 * Gold mines

Gold is probably the most important resource in the game: all infantry units, all aircraft, most warships and most upgrades need it in droves. Only six Citizens can gather from one gold mine which is usually not enough; gold mines are frequent targets of expansion so guard yours well.

Gold mines have a capacity of 300000 which means they never run out during all but the most drawn-out stalemates.

Iron
Gained from:
 * Iron mines

Iron is just slightly less important than gold: cavalry, siege weapons, all aircraft and some warships require it. All the same rules as with gold apply here too: iron mines are targets of expansion since six Citizens on one mine is not enough.

Iron mines have a capacity of 300000 so they never run out.