diff --git a/Sources/AOC2018/day12.swift b/Sources/AOC2018/day12.swift
new file mode 100644
index 0000000..3bdcbfa
--- /dev/null
+++ b/Sources/AOC2018/day12.swift
@@ -0,0 +1,175 @@
+//
+// Advent of Code 2018 "Day 12: Subterranean Sustainability"
+//
+
+import Foundation
+
+class Sustainability {
+    var condition = ""
+    var pattern: [String: String] = [:]
+    var keyLength = 0
+    var paddingLength = 0
+
+    // Supply test data in the form of a String
+    init(withString noteList: String, andInitialCondition initial: String, padding pad: Int) {
+        paddingLength = pad
+        let padding = String(Array(repeating: ".", count: paddingLength))
+        condition = padding + initial + padding
+        let noteStrings = noteList.components(separatedBy: "\n")
+        parseData(withNotes: noteStrings)
+   }
+
+    func parseData(withNotes noteArray: [String]) {
+        for oneNote in noteArray {
+            var line = oneNote
+            line = line.replacingOccurrences(of: " => ", with: " ")
+            let data = line.components(separatedBy: " ")
+            guard data.count == 2 else { print("Error"); return }
+            pattern[data[0]] = data[1]
+            keyLength = data[0].count
+        }
+    }
+
+    func nextGeneration(with previous: String) -> String {
+        var nextGen = String(Array(repeating: ".", count: keyLength/2))
+        for i in 0..<previous.count-keyLength {
+            let start = previous.index(previous.startIndex, offsetBy: i)
+            let end = previous.index(previous.startIndex, offsetBy: i + keyLength)
+            let range = start..<end
+            let substr = String(previous[range])
+            if  pattern[substr] == nil {
+                nextGen += "."
+            } else {
+                nextGen = nextGen + pattern[substr]!
+            }
+        }
+        nextGen += String(Array(repeating: ".", count: keyLength/2+1))
+        return nextGen
+    }
+
+    func getValue(of plants: String) -> Int {
+        var value = 0
+        let start = -paddingLength
+        for i in 0..<plants.count {
+            let pot = start + i
+            value += plants[i] == "#" ? pot : 0
+        }
+        return value
+    }
+}
+
+class Day12: AOCDay {
+    lazy var tests: (() -> ()) = day12Tests
+    lazy var final: (() -> ()) = day12Final
+
+    func testSustainabilityInit() {
+        let _ = Sustainability(withString: testNotes, andInitialCondition: testInitialState, padding: 15)
+    }
+
+    func testNextGeneration() {
+        let sus = Sustainability(withString: testNotes, andInitialCondition: testInitialState, padding: 15)
+        var gen = sus.condition
+        print(" 0", terminator: ": ")
+        print("\(gen)")
+        for i in 1...20 {
+            print(NSString(format:"%2d", i), terminator: ": ")
+            gen = sus.nextGeneration(with: gen)
+            print("\(gen)")
+        }
+    }
+
+    func testGetVaue() {
+        let sus = Sustainability(withString: testNotes, andInitialCondition: testInitialState, padding: 15)
+        var gen = sus.condition
+        for _ in 1...20 {
+            gen = sus.nextGeneration(with: gen)
+        }
+        let value = sus.getValue(of: gen)
+        XCTAssertEqual(test: "testGetVaue", withExpression: (value  == 325))
+    }
+
+    func day12Tests() {
+        testSustainabilityInit()
+        testNextGeneration()
+        testGetVaue()
+    }
+
+    func day12Final() {
+        let sus = Sustainability(withString: finalNotes, andInitialCondition: finalInitialState, padding: 200)
+        var gen = sus.condition
+        for _ in 1...20 {
+            gen = sus.nextGeneration(with: gen)
+        }
+        let value = sus.getValue(of: gen)
+        print("Answer to part 1 is: \(value)")
+
+
+        let limitCount = 150
+        var inc = 0 //40
+        gen = sus.condition
+        for _ in 1...limitCount {
+            let preValue = sus.getValue(of: gen)
+            gen = sus.nextGeneration(with: gen)
+            let postValue = sus.getValue(of: gen)
+            inc = postValue - preValue
+        }
+        let valLimit = sus.getValue(of: gen)
+        let generations = 50000000000 - limitCount
+        let answer = generations * inc + valLimit
+        print("Answer to part 2 is: \(answer)")
+    }
+}
+
+var testInitialState = "#..#.#..##......###...###"
+var testNotes = """
+    ...## => #
+    ..#.. => #
+    .#... => #
+    .#.#. => #
+    .#.## => #
+    .##.. => #
+    .#### => #
+    #.#.# => #
+    #.### => #
+    ##.#. => #
+    ##.## => #
+    ###.. => #
+    ###.# => #
+    ####. => #
+    """
+
+var finalInitialState = "##.......#.######.##..#...#.#.#..#...#..####..#.##...#....#...##..#..#.##.##.###.##.#.......###....#"
+var finalNotes = """
+    .#### => .
+    ....# => .
+    ###.. => .
+    ..#.# => .
+    ##### => #
+    ####. => .
+    #.##. => #
+    #.#.# => .
+    ##.#. => #
+    .###. => .
+    #..#. => #
+    ###.# => .
+    #.### => .
+    ##... => #
+    .#.## => .
+    ..#.. => .
+    #...# => #
+    ..... => .
+    .##.. => .
+    ...#. => .
+    #.#.. => .
+    .#..# => #
+    .#.#. => .
+    .#... => #
+    ..##. => .
+    #..## => .
+    ##.## => #
+    ...## => #
+    ..### => #
+    #.... => .
+    .##.# => #
+    ##..# => #
+    """
diff --git a/Sources/AOC2018/main.swift b/Sources/AOC2018/main.swift
index c41e52e..c1adc7e 100644
--- a/Sources/AOC2018/main.swift
+++ b/Sources/AOC2018/main.swift
@@ -5,7 +5,7 @@
 import Foundation
 
 let showTests = true
-let onlyOneDay = 11
+let onlyOneDay = 0
 var allTests: [(() -> ())] = []
 var allFinal: [(() -> ())] = []
 
@@ -23,6 +23,7 @@ allTests.append(Day08().tests)
 allTests.append(Day09().tests)
 allTests.append(Day10().tests)
 allTests.append(Day11().tests)
+allTests.append(Day12().tests)
 
 // Compile list of Answers
 allFinal.append(Day01().final)
@@ -36,6 +37,7 @@ allFinal.append(Day08().final)
 allFinal.append(Day09().final)
 allFinal.append(Day10().final)
 allFinal.append(Day11().final)
+allFinal.append(Day12().final)
 
 if onlyOneDay > 0 {
     print("\nDay \(onlyOneDay)")