diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index 6b92aab..084b123 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -18,4 +18,5 @@
                (:file "day7")
                (:file "day8")
                (:file "day9")
-               (:file "day10")))
+               (:file "day10")
+               (:file "day11")))
diff --git a/day11.lisp b/day11.lisp
new file mode 100644
index 0000000..dd2ffcb
--- /dev/null
+++ b/day11.lisp
@@ -0,0 +1,55 @@
+;;;; day11.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day11-grid-value (x y serial)
+  (- (mod (floor
+           (* (+ x 10)
+              (+ (* (+ x 10) y)
+                 serial))
+           100)
+          10)
+     5))
+
+(defun day11-compute-grid (serial)
+  (loop :with grid := (make-array '(300 300) :element-type 'fixnum)
+        :for x :below 300
+        :do (loop :for y :below 300
+                  :do (setf (aref grid x y) (day11-grid-value (1+ x) (1+ y) serial))
+                  :when (> x 0) :do (incf (aref grid x y) (aref grid (1- x) y))
+                  :when (> y 0) :do (incf (aref grid x y) (aref grid x (1- y)))
+                  :when (and (> x 0) (> y 0)) :do (decf (aref grid x y) (aref grid (1- x) (1- y))))
+        :finally (return grid)))
+
+(defun day11-grid-sum (grid x y &optional (size 3))
+  (let ((delta (1- size)))
+    (- (aref grid (+ x delta) (+ y delta))
+       (if (> y 0) (aref grid (+ x delta) (1- y)) 0)
+       (if (> x 0) (aref grid (1- x) (+ y delta)) 0)
+       (- (if (and (> x 0) (> y 0)) (aref grid (1- x) (1- y)) 0)))))
+
+(defun day11-max-square (grid size)
+  (loop :with max-x := nil
+        :with max-y := nil
+        :with max := (* -5  size size)
+        :for x :from 1 :upto (- 301 size)
+        :do (loop :for y :from 1 :upto (- 301 size)
+                  :for sum := (day11-grid-sum grid (1- x) (1- y) size)
+                  :when (> sum max) :do (setf max sum max-x x max-y y))
+        :finally (return (list max-x max-y max))))
+
+(defun day11 (&optional (serial 9306))
+  (let ((grid (day11-compute-grid serial)))
+    (destructuring-bind (x y charge) (day11-max-square grid 3)
+      (format t "The highest charged 3x3 square has its top-left corner at (~a, ~a) and a charge of ~a.~%"
+              x y charge))
+    (loop
+      :with max-charge := 0
+      :with max-x := nil
+      :with max-y := nil
+      :with max-size := nil
+      :for size :from 1 :to 300
+      :for (x y charge) := (day11-max-square grid size)
+      :when (> charge max-charge)
+      :do (setf max-charge charge max-x x max-y y max-size size)
+      :finally (format t "The square with the highest charge lies at (~a, ~a), is ~ax~:*~a and with a total charge of ~a.~%Solution: ~0@*~a,~a,~a" max-x max-y max-size max-charge))))