I was thinking of a scenario I saw, and was wondering how the rules would work.  The PC got into an encounter, and led off the initiative order.  They weren't sure what should be done (game-wise they were new to the area), so he was looking for guidance from the lead NPC who was their guide, so he readied an action where if the NPC moved away (retreated), he would follow him.  Before the NPCs turn though, a creature approached the NPC and attacked him.  Could he instead have readied an action like this?

"If an enemy moves within melee range of me, I will attack it, but if the NPC moves first, I will follow him in retreat."

Then, you would do only one of these, whichever comes first?  Or can it only truly be one thing only you can be ready for?  Based on real life, I should be able to do both, but not sure what is RAW.  

Thank you.

You can only set one trigger and one action to take in response.  Your example is setting two triggers (the NPC getting attacked, and the NPC moving), each with a separate action in response (attacking the creature or following the NPC), so this is technically not allowed.  You also cannot move and take an action in response to a trigger, the text says you can do either/or.

Part of the risk of using the Ready action is that your trigger may not come to pass;  you should set your trigger to allow the greatest possibility of it happening, or to mitigate the worst result. Reactions are meant to be so quick as to be nearly instant; so you don't have the time to deliberate between options or choose a course of action after the trigger occurs (other than to ignore the trigger if you want), and you can't consider two triggers at once.

Thank you!  I think that is one of the situations where I need to recognize more the flow of the action.  While it plays out as being one at a time, in reality everything is happening at once.  Based on that, yes you would only be realistically be able to do one thing.  I appreciate the response!

It says "decide what perceivable circumstance will trigger your reaction". The whole section doesn't really talk about only limited to one trigger or one circumstance.

Also, "either" "or" doesn't really make it two different circumstance if we look at the logic-gate point of view. It is one function with two inputs and one output.

The devs have revealed that their decision to not allow people to simply delay their turn was based entirely on the goal of keeping combat simple. Unfortunately the cost of that simplicity is to not be able to react realistically in situations like this.

For what it's worth, my group has discussed having a one-time chance - immediately after initiative is rolled - for people to move themselves down to a specific spot in the initiative order. I think limiting the ability to delay your whole turn to this one moment preserves the rule simplicity while avoiding the weird situation where the prepared guy absolutely has to act before the unprepared guy.

But we also allow brief communication off-turn. If I were the PC in OP's suggestion, I'd have looked at the NPC and asked something like, "Fight or flee?" You'd be a pretty stingy DM to not allow a single word uttered in response.

You can only ready 1 type of action/movement and only have 1 trigger (circumstance and trigger are only ever refered to in the singular throughout the ready rules).

Quote from Frozen_Fruit >>

It says "decide what perceivable circumstance will trigger your reaction". The whole section doesn't really talk about only limited to one trigger or one circumstance.

Also, "either" "or" doesn't really make it two different circumstance if we look at the logic-gate point of view. It is one function with two inputs and one output.

 Circumstance is singular, as is the entire section on Readying.  Grammar and word choice matter.  If you could ready for multiple triggers, that eliminates the need for a trigger...you could just keep making triggers until you could react to anything.  Logic-gate has nothing to do with it...it’s simply the rule as written 

Quote from Frozen_Fruit >>

It says "decide what perceivable circumstance will trigger your reaction". The whole section doesn't really talk about only limited to one trigger or one circumstance.

Also, "either" "or" doesn't really make it two different circumstance if we look at the logic-gate point of view. It is one function with two inputs and one output.

 From a logic-gate point of view:

AND can be used to defined a set of criteria for a single circumstance;

OR can be used to defined a collection of separate circumstances.