Hi Rich. Good questions (and excellently formatted). Usually I try to lead a person to the right answer, but here I'm just going to write it in full because your questions are more conceptual and you have clearly tried to get it right. Let me see if I can take you step by step through my logic when answering this challenge.

1) Why is there a question mark on the value of the key/value pair and also on the outside of the type declaration [String:String?]? (I understand the one inside, but both?)

So this is fundamental to the challenge and to more complicated coding. Here there are several possible ways that you could fail to retrieve a value. We are looking at two in particular: 1) You could retrieve a dictionary without data [:]?. 2) You could retrieve a dictionary without your data (you don't have the key) [:?].

2) If I am using the guard keyword/clause, why do I still have to add a question mark to the data variable which holds the dictionary containing the optional value guard let key = data?["someString"]

You don't. At least, in the way that I wrote it you don't.

3) This code below works for the treehouse compiler but in xcode, the second guard statement guard let value = key brings up a value was not defined, consider replacing with boolean fix?

So for this last one, you are writing "guard let value". This tells the compiler that you are declaring a new property: "value". This property has not been initialized and, in this case, there is no real reason to declare a new value.

enum ParserError: ErrorType {
    case EmptyDictionary
    case InvalidKey
}

struct Parser {
    var data: [String : String?]?

    func parse() throws {
        guard data != nil else {
            throw ParserError.EmptyDictionary
        }
        guard data!.keys.contains("someKey") else {
            throw ParserError.InvalidKey
        }
    }
}

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)

So this is how I structured mine. I checked to see if the dictionary had no data (guard data != nil). Then I checked to see if the key to the data contained "someKey". The methods "keys.contains" are built-in, I did not write those methods myself.

Additional point of interest. You will see that I force unwrapped "data" in the second throw. The reason I did this was because I already checked to see if the value of dictionary was nil using the first throw. Therefore, because it passed the check, I thought it would be safe to force unwrap. I'm not experienced enough to say whether that will always be the right choice, but I hope you appreciate my logic.

Thanks and sorry for the long explanation!