An assist is the action of helping an allied champion kill someone. You can either score an assist by hitting (without killing) the champion in the last 10 seconds before their death, or by contributing passively during this period of time.
When a champion is killed, the killer gets a certain amount of gold, while 50% of that amount is divided up among everyone who got an assist on it.
Secondary System
- Champions also gain assists on kills if they:
- Contributed to the battle via active abilities and basic attacks, for example stuns or knockups, whether or not they dealt damage to that enemy.
- Debuffed an enemy, even if that enemy was immune to the damage.
- Supporting a champion that contributed to the kill, e.g. healing or buffing an allied champion.
- Killing an enemy using
Trundle's 
Pillar of Ice or 
Veigar's 
Event Horizon (1 true damage each)
- Champions will not gain assists if they:
- ONLY use blind, reveal, or attack speed slow, without also contributing in any of the above ways.
Triggering effects with assists
- Champion abilities, items and runes that trigger an effect with an assist can be seen on the takedown page.