diff --git a/2021/5/go.mod b/2021/5/go.mod
new file mode 100644
index 0000000..e282ece
--- /dev/null
+++ b/2021/5/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/5
+
+require AoC/2021/common v0.0.0
+
+require github.com/joho/godotenv v1.4.0 // indirect
+
+replace AoC/2021/common v0.0.0 => ../common
+
+go 1.17
diff --git a/2021/5/go.sum b/2021/5/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/5/go.sum
@@ -0,0 +1,2 @@
+github.com/joho/godotenv v1.4.0 h1:3l4+N6zfMWnkbPEXKng2o2/MR5mSwTrBih4ZEkkz1lg=
+github.com/joho/godotenv v1.4.0/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
diff --git a/2021/5/improved.go b/2021/5/improved.go
new file mode 100644
index 0000000..ee63d26
--- /dev/null
+++ b/2021/5/improved.go
@@ -0,0 +1,101 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"regexp"
+	"strconv"
+)
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func findIntersection(diagonal bool) func (*bufio.Scanner) int {
+	return func (input *bufio.Scanner) int {
+		size := 1000
+		diagram := make([][]int, size)
+		for y := 0; y < size; y++ {
+			diagram[y] = make([]int, size)
+		}
+
+		re := regexp.MustCompile("[0-9]+")
+		for input.Scan() {
+			text := input.Text()
+			numbers := re.FindAllString(text, -1)
+
+			if len(numbers) != 4 {
+				panic("Did not find 4 numbers")
+			}
+
+			x1, _ := strconv.Atoi(numbers[0])
+			y1, _ := strconv.Atoi(numbers[1])
+			x2, _ := strconv.Atoi(numbers[2])
+			y2, _ := strconv.Atoi(numbers[3])
+
+			// Optionally skip lines that are not horizontal or vertical
+			if !diagonal && !(x1 == x2 || y1 == y2) {
+				continue
+			}
+
+			dx := 1
+			dy := 1
+			length := max(max(x1, x2) - min(x1, x2) + 1, max(y1, y2) - min(y1, y2) + 1)
+
+			if x1 == x2 {
+				dx = 0
+			} else if x1 > x2 {
+				dx = -dx
+			}
+
+			if y1 == y2 {
+				dy = 0
+			} else if y1 > y2 {
+				dy = -dy
+			}
+
+			for i := 0; i < length; i++ {
+				diagram[y1 + i*dy][x1 + i*dx]++
+			}
+		}
+
+		points := 0
+		for y := 0; y < size; y++ {
+			for x := 0; x < size; x++ {
+				if diagram[y][x] > 1 {
+					points++
+				}
+			}
+		}
+
+		return points
+	}
+}
+
+func main() {
+	aoc := aoc.New(2021, 5)
+
+	aoc.Test(`0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2`, []int{5, 12})
+
+	aoc.Solution(1, findIntersection(false))
+	aoc.Solution(2, findIntersection(true))
+}
diff --git a/2021/5/main.go b/2021/5/main.go
new file mode 100644
index 0000000..a85b58a
--- /dev/null
+++ b/2021/5/main.go
@@ -0,0 +1,129 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"regexp"
+	"strconv"
+)
+
+type line struct {
+	x1, y1, x2, y2 int
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func findIntersection(diagonal bool) func (*bufio.Scanner) int {
+	return func (input *bufio.Scanner) int {
+		var lines []line
+		var xmax, ymax int
+
+		re := regexp.MustCompile("[0-9]+")
+		for input.Scan() {
+			text := input.Text()
+			numbers := re.FindAllString(text, -1)
+
+			if len(numbers) != 4 {
+				panic("Did not find 4 numbers")
+			}
+
+			var coords [4]int
+			for i, n := range numbers {
+				c, err := strconv.Atoi(n)
+				if err != nil {
+					panic(err)
+				}
+				coords[i] = c
+			}
+
+			xmax = max(xmax, max(coords[0], coords[2]))
+			ymax = max(ymax, max(coords[1], coords[3]))
+
+			// Only consider horizontal and vertical line if diagonal is false
+			if diagonal || coords[0] == coords[2] || coords[1] == coords[3] {
+				lines = append(lines, line{coords[0], coords[1], coords[2], coords[3]})
+			}
+		}
+
+		diagram := make([][]int, ymax+1)
+		for y := 0; y <= ymax; y++ {
+			diagram[y] = make([]int, xmax+1)
+		}
+
+		for _, a := range lines {
+			// Vertical line
+			if a.x1 == a.x2 {
+				for y := min(a.y1, a.y2); y <= max(a.y1, a.y2); y++ {
+					diagram[y][a.x1]++
+				}
+				continue
+			}
+
+			// Horizontal line
+			if a.y1 == a.y2 {
+				for x := min(a.x1, a.x2); x <= max(a.x1, a.x2); x++ {
+					diagram[a.y1][x]++
+				}
+				continue
+			}
+
+			// Otherwise we have a diagonal line
+			length := max(a.x1, a.x2) - min(a.x1, a.x2) + 1
+			for i := 0; i < length; i++ {
+				x := a.x1 + i
+				if (a.x1 > a.x2) {
+					x = a.x1 - i
+				}
+
+				y := a.y1 + i
+				if (a.y1 > a.y2) {
+					y = a.y1 - i
+				}
+
+				diagram[y][x]++
+			}
+		}
+
+
+		points := 0
+		for y := 0; y <= ymax; y++ {
+			for x := 0; x <= xmax; x++ {
+				if diagram[y][x] > 1 {
+					points++
+				}
+			}
+		}
+
+		return points
+	}
+}
+
+func main() {
+	aoc := aoc.New(2021, 5)
+
+	aoc.Test(`0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2`, []int{5, 12})
+
+	aoc.Solution(1, findIntersection(false))
+	aoc.Solution(2, findIntersection(true))
+}