Just returned to playing after many years away, am dming the premade stuff from the essentials kit for my family, all good. 

One if them is a ranger who likes firing into combat, and I cannot for the life of me find the rules for this I can find the rules on using a ranged weapon whilst engaged etc but not what happens if you fire into a melee.

I remember many editions ago this was a complex thing with chance of hitting the wrong person in the confusion on melee I want to be sure we're playing right to 5e rules so where do I look or what is the rule.

Thank you

Ahhh the old 2nd edition firing into combat was... a nightmare and I loved it.

The 5th edition ruleset doesn't have anything like that. Obviously, there are cover rules and another creature in the path of the ammunition would provide cover making the AC of the target higher, but that is it.

Quote from Orcidas >>

Just returned to playing after many years away, am dming the premade stuff from the essentials kit for my family, all good. 

One if them is a ranger who likes firing into combat, and I cannot for the life of me find the rules for this I can find the rules on using a ranged weapon whilst engaged etc but not what happens if you fire into a melee.

I remember many editions ago this was a complex thing with chance of hitting the wrong person in the confusion on melee I want to be sure we're playing right to 5e rules so where do I look or what is the rule.

Thank you

1. All creatures provide half cover to all other creatures. 
   
   1. That is, all creatures with a statblock - creatures too tiny to warrant one, like gnats, feel free to ignore.  However, a rat will provide light cover to a goliath.
   2. There's an optional rule in the DMG for hitting cover by accident, but the optional rule doesn't robustly handle figuring out which of multiple pieces of cover is hit when you are getting cover from overlapping sources.  I've played with this rule, and so far every DM I've had has settled the issue by hitting the closest valid target to the attacker.
2. When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn't incapacitated.

Thank you that's more than enough for me to work with.

There is also an optional rule in the Dungeon Master Guide called Hitting Cover that has similarities to the AD&D 2nd Edition rule Firing Into Combat i believe;.

Hitting Cover: When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack. First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.