See the experience required for each level here.

Experience For Non-partied Players

Characters Under Level 25

If there is only a small level difference between the monster and yourself (5 levels or less), you receive 100% of that monster's experience points.

You will receive fewer experience points as the level difference increases. This is the table used to adjust the experience gained:

mlvl-clvl	exp %
>10     	2
10      	2
9       	15
8       	36
7       	68
6       	88
1 to 5  	100
-5 to 0 	100
-6      	81
-7      	62
-8      	43
-9      	24
-10     	5
<-10     	5

If the monster is 10 levels or more below your level, you receive 5% of the monster's experience points. If the monster is 10 levels or more above your level, you receive 2% of the monster's experience points.

Characters Level 25-99

For any monster above your level, you get EXP*(Player Level / Monster Level).

For monsters between 5-10 levels below you, you receive between 100% and 5% experience.

lvl difference	exp %
6  	    	81
7	      	62
8	      	43
9	      	24
10		5
> 10	        5

If the monster is 10 levels or more below your level, you receive 5% of the monster's experience points.

Level Experience Level Experience Level Experience 70 95.31% 80 48.44% 90 5.96% 71 90.63% 81 43.75% 91 4.49% 72 85.94% 82 39.06% 92 3.42% 73 81.25% 83 34.38% 93 2.54% 74 76.56% 84 29.69% 94 1.95% 75 71.88% 85 25.00% 95 1.46% 76 67.19% 86 18.75% 96 1.07% 77 62.50% 87 14.06% 97 0.78% 78 57.81% 88 10.55% 98 0.59% 79 53.13% 89 7.91%

As you can see, high-level characters receive only a small amount of experience for killing low-level monsters. Similarly, low-level characters only get a small amount of experience for killing high-level monsters.

What happens when two neutral players attack the same monster?
Whichever player makes the killing blow will get the experience.

Experience In a Party
The total experience earned for killing a monster is increased by 35% when a party member of the killer is in the same named area, defined as the "level" on the Automap. Then the pool is divided as explained below:

The total experience earned for killing a monster is divided evenly among all party members. Each member's share of the experience is equal to the member's level divided by the sum of all members' levels. This Experience Share is then multiplied by another percentage, calculated on a sliding scale that is based on the difference between your character's level and the monster's level, regardless of whether you are in a party or not. The sliding scale only applies if the difference is between 5 and 10 levels. With a small difference (5 levels or less), you receive 100% of your Experience Share. With a large difference (10 levels or more), you will receive 5% of your Experience Share. As a result, high-level characters receive only a small amount of experience for killing low-level monsters. Similarly, low-level characters only get a small amount of experience for killing high-level monsters. This is so that a low-level character running in a high-level party won't receive ridiculous amounts of experience fighting high-level monsters that only his party-mates can destroy.

Finally, only those party members within 2 screens of the monster death receive experience.

Do you get any extra or bonus experience for being in a party?
Yes.

Experience Loss In Nightmare and Hell Difficulty
In Nightmare and Hell difficulties, you lose experience each time you die. However, you will never lose a level. Your experience loss (5% in Nightmare, and 10% in Hell) is applied to the difference in experience points between your current level and the next level. For example, If you reached level X with 1,000,000 experience and you would reach level X+1 at 2,000,000 experience, then the penalty is 5% or 10% of (2,000,000 - 1,000,000): 5% or 10% of 1,000,000 points.

You can regain some experience in Nightmare and Hell Difficulty by recovering your corpse
In Nightmare and Hell difficulty settings, whenever your character dies, he or she suffers a loss of gold and experience points. In games of Diablo II: Lord of Destruction, if you recover your corpse at the location of its demise, you can regain 75% of the experience points you lost. If, however, you choose to 'Save and Exit' out of your current game in order to restart and recover your body in town, you will not regain any of your lost experience.

Do you get any experience for killing monsters that were resurrected by other monsters?
No. You only get experience from killing a monster the first time.

Do Hirelings and Minions steal your experience?
No. You don't lose any experience for using Hirelings. In fact, you gain experience, because the experience points from their kills are added to your own experience point totals.

Monster Experience In Multiplayer Games
Monsters' experience value and Hit Points will change with the number of players in the game. When more people join a game, the monsters become harder, and as they leave the game, the monsters become easier. For the ultimate challenge, fight solo against monsters on Hell difficulty in an 8-player game. In multiplayer, when a monster is created, its Hit Points are multiplied by the number of players in the game: (Life = Hit Points * (Number of Players + 1) / 2). The monster's base experience value is determined by the following formula:

Experience = X * (n + 1) / 2

X = base experience
n = # of players in the game

Upping the Party Members in Single Player and TCP/IP
Typing "/players X", where X is a number between the current number of players and the maximum (8), in the message box will set the effective number of players in the game (in single player, open Battle.net and TCP/IP games only). This will increase the Monster Life, Experience, and Item Drops. If this command sets the number of players higher than the current amount of players in the game, the added amount of players will be considered to be just in the game, and not part of your party.