diff --git a/src/main/scala/com/rockthejvm/part4coordination/Refs.scala b/src/main/scala/com/rockthejvm/part4coordination/Refs.scala
new file mode 100644
index 0000000..bb29782
--- /dev/null
+++ b/src/main/scala/com/rockthejvm/part4coordination/Refs.scala
@@ -0,0 +1,160 @@
+package com.rockthejvm.part4coordination
+
+import cats.effect.{IO, IOApp, Ref}
+import com.rockthejvm.utils._
+
+import javax.sound.midi.SysexMessage
+import scala.concurrent.duration._
+
+object Refs extends IOApp.Simple {
+
+  // ref = purely functional atomic reference
+  val atomicMol: IO[Ref[IO, Int]] = Ref[IO].of(42)
+  val atomicMol_v2: IO[Ref[IO, Int]] = IO.ref(42)
+
+  // modifying is an effect
+  val increasedMol: IO[Unit] = atomicMol.flatMap { ref =>
+    ref.set(43) // thread-safe
+  }
+
+  // obtain a value
+  val mol = atomicMol.flatMap { ref =>
+    ref.get // thread-safe
+  }
+
+  val gsMol: IO[Int] = atomicMol.flatMap { ref =>
+    ref.getAndSet(43)
+  } // gets the old value, sets the new one
+
+  // updating with a function
+  val fMol: IO[Unit] = atomicMol.flatMap { ref =>
+    ref.update(value => value * 10)
+  }
+
+  val updatedMol: IO[Int] = atomicMol.flatMap { ref =>
+    ref.updateAndGet(value => value * 10) // get the new value
+    // can also use getAndUpdate to get the OLD value
+  }
+
+  // modifying with a function returning a different type
+  val modifiedMol: IO[String] = atomicMol.flatMap { ref =>
+    ref.modify(value => (value * 10, s"my current value is $value"))
+  }
+
+  // why: concurrent + thread-safe reads/writes over shared values, in a purely functional way
+
+  import cats.syntax.parallel._
+  def demoConcurrentWorkImpure(): IO[Unit] = {
+    var count = 0
+
+    def task(workload: String): IO[Unit] = {
+      val wordCount = workload.split(" ").length
+      for {
+        _ <- IO(s"Counting words for '$workload': $wordCount'").debug
+        newCount <- IO(count + wordCount)
+        _ <- IO(s"New total: $newCount").debug
+        _ <- IO(count += wordCount)
+      } yield ()
+    }
+
+    List("I love Cats Effect", "This ref thing is useless", "Daniel writes a lot of code")
+      .map(task)
+      .parSequence
+      .void
+  }
+  /*
+    Drawbacks:
+    - hard to read/debug
+    - mix pure/impure code
+    - NOT THREAD SAFE
+   */
+
+  def demoConcurrentWorkPure(): IO[Unit] = {
+    def task(workload: String, total: Ref[IO, Int]): IO[Unit] = {
+      val wordCount = workload.split(" ").length
+
+      for {
+        _ <- IO(s"Counting words for '$workload': $wordCount'").debug
+        newCount <- total.updateAndGet(currentCount => currentCount + wordCount)
+        _ <- IO(s"New total: $newCount").debug
+      } yield ()
+    }
+
+    for {
+      initialCount <- Ref[IO].of(0)
+      _ <- List("I love Cats Effect", "This ref thing is useless", "Daniel writes a lot of code")
+        .map(string => task(string, initialCount))
+        .parSequence
+    } yield ()
+  }
+
+  /**
+   * Exercise
+   */
+  def tickingClockImpure(): IO[Unit] = {
+    var ticks: Long = 0L
+    def tickingClock: IO[Unit] = for {
+      _ <- IO.sleep(1.second)
+      _ <- IO(System.currentTimeMillis()).debug
+      _ <- IO(ticks += 1) // not thread safe
+      _ <- tickingClock
+    } yield ()
+
+    def printTicks: IO[Unit] = for {
+      _ <- IO.sleep(5.seconds)
+      _ <- IO(s"TICKS: $ticks").debug
+      _ <- printTicks
+    } yield ()
+
+    for {
+      _ <- (tickingClock, printTicks).parTupled
+    } yield ()
+  }
+
+  def tickingClockPure(): IO[Unit] = {
+    def tickingClock(ticks: Ref[IO, Int]): IO[Unit] = for {
+      _ <- IO.sleep(1.second)
+      _ <- IO(System.currentTimeMillis()).debug
+      _ <- ticks.update(_ + 1) // thread safe effect
+      _ <- tickingClock(ticks)
+    } yield ()
+
+    def printTicks(ticks: Ref[IO, Int]): IO[Unit] = for {
+      _ <- IO.sleep(5.seconds)
+      t <- ticks.get
+      _ <- IO(s"TICKS: $t").debug
+      _ <- printTicks(ticks)
+    } yield ()
+
+    for {
+      tickRef <- Ref[IO].of(0)
+      _ <- (tickingClock(tickRef), printTicks(tickRef)).parTupled
+    } yield ()
+  }
+
+  def tickingClockWeird(): IO[Unit] = {
+    val ticks = Ref[IO].of(0) // IO[ref]
+
+    def tickingClock: IO[Unit] = for {
+      t <- ticks // ticks will give you a NEW Ref
+      _ <- IO.sleep(1.second)
+      _ <- IO(System.currentTimeMillis()).debug
+      _ <- t.update(_ + 1) // thread safe effect
+      _ <- tickingClock
+    } yield ()
+
+    def printTicks: IO[Unit] = for {
+      t <- ticks // ticks will give you a NEW Ref
+      _ <- IO.sleep(5.seconds)
+      currentTicks <- t.get
+      _ <- IO(s"TICKS: $currentTicks").debug
+      _ <- printTicks
+    } yield ()
+
+    for {
+      _ <- (tickingClock, printTicks).parTupled
+    } yield ()
+  }
+
+  override def run = tickingClockWeird()
+}