You were in the middle of something. Maybe Chrome had been open for days, or you were editing a big Lightroom project alongside a dozen other apps. Then a dialog appeared: "Your system has run out of application memory." Below it, a list of apps. A close button. Your heart sank a little.
Take a breath. Your Mac is not broken. This message is macOS being transparent about something that happens to every Mac eventually: you ran out of RAM for what you were asking the machine to do. Understanding why that happened, and what the message actually means, is the fastest way to stop it recurring.
What does this error message actually mean?
Your Mac has a fixed amount of RAM (short for random-access memory), the fast working space where it keeps everything you currently have open. When you open an app, macOS loads it into RAM. When you open a second app, a third, a browser with fifty tabs, a video editor and Slack at the same time, all of those things compete for the same pool of memory.
macOS is clever about managing this. It compresses less-used bits of memory to make them smaller, and it can spill overflow onto your internal storage using something called swap space. Swap is much slower than RAM, but it buys time. Under normal conditions you never notice any of this happening.
When even compressed memory and swap are exhausted, macOS has no more room. At that point it makes a choice: it can either crash the entire system, or it can close some apps in an orderly way to free space. It chooses the latter. The dialog you see is it telling you which apps it wants to close, and asking your permission before it does.
Why does this happen?
There are three common causes, and one of them usually explains what you saw.
Too many apps open at once. This is the most common cause. Chrome is the single biggest offender because each tab runs in its own process, and a browser with fifty tabs can use several gigabytes of RAM on its own. Add Slack (which is also built on web technology and carries a similar footprint), Photoshop, Spotify, and a few smaller apps, and even 16 GB of RAM can fill up.
A memory leak in one app. A memory leak is when an app grabs RAM and then never lets it go, even when it no longer needs it. The app just keeps growing. Slack after weeks of uptime without a restart is a well-known example. Mail.app has shipped leaky updates in some macOS versions. A leaking app can quietly consume several gigabytes over the course of a day without you noticing until the error appears. You can spot this by opening Activity Monitor (search for it with Spotlight) and clicking the Memory tab. Sort by "Memory" to see which app is using the most. If one app is showing numbers far higher than the others, that is worth restarting.
Not enough RAM for your actual workflow. Sometimes there is no leak and you didn't have an unusual number of tabs open. You just genuinely need more RAM than your Mac has. This is more likely on an 8 GB machine running a heavy creative workflow: 4K video editing, large Photoshop files, or complex Lightroom catalogues push memory hard. If you see this error regularly, Apple's Activity Monitor guide explains how to use the memory pressure graph to judge whether more RAM would actually help. We also cover the pressure graph in detail in our post on what memory pressure on Mac means.
Is this a sign my Mac is failing?
No. Full stop.
This error has nothing to do with physical hardware failure. RAM itself does not wear out the way a hard drive does. What you are seeing is a software resource limit being reached, not a component breaking down.
It is worth saying clearly because the dialog looks serious, and it is easy to spiral into worrying about an expensive repair or replacement. The Mac equivalent of a hardware memory problem would be random crashes, apps freezing without warning, or corrupted files. A clean dialog box asking which apps to close is the opposite of that. It is macOS behaving exactly as designed.
What should I do when I see it?
First: save anything important before you click anything. When you see the dialog, your first move should be Command-Tab through your open apps and save any unsaved documents, especially in the apps listed in the dialog. Once macOS closes an app via its kill mechanism (which engineers call jetsam), unsaved work in that app is gone.
Second: let macOS close the apps it has suggested. It has made a reasonable guess at what is lowest priority. Once the apps are closed, your pressure should drop and the system will stabilise.
Third: if you can identify the likely culprit (a browser with many tabs, an app that has been open for weeks), restart just that app. A restart clears any leaked memory and gives it a fresh start.
For the full list of practical steps, including how to use Activity Monitor to find the leaking app and how to keep this from happening again, read our companion post on how to fix the "out of application memory" error on Mac. That post covers the action steps in detail; this one focuses on what the error means.
How do I stop it happening again?
The honest answer is that prevention mostly comes down to habits, not tricks.
Close tabs you're not reading. Browser tabs are the single most effective thing you can reduce. A tab you opened three days ago and haven't looked at is still consuming memory. One well-known rule among developers is to treat tabs like an inbox: either read it now or close it.
Restart apps weekly, not just your whole Mac. Long-running apps, especially Slack, Chrome, and email clients, tend to grow in memory over time. Quitting and relaunching them costs thirty seconds and can reclaim gigabytes.
Keep a bit of storage free. macOS uses your internal drive for swap space, so a nearly full disk leaves the system less room to breathe. Keeping at least 10-15 GB free is a reasonable target. If storage is tight, our post on how to free up RAM on Mac covers both memory and storage-adjacent steps that help.
Use Activity Monitor as a quick check. If your Mac feels sluggish and you're wondering whether it's about to show you the error, open Activity Monitor and look at the Memory Pressure graph at the bottom. Red pressure sustained for more than a few minutes is the warning sign. Acting before the dialog appears is better than reacting after it does. Stopping memory pressure before it hits red is what Shiny does: one click from the menu bar asks macOS to release inactive memory, closes orphaned helper processes, and pauses idle apps before the situation becomes critical. $4.99 once, no subscription.
You can also learn to use Activity Monitor more confidently with our walkthrough of how to use Activity Monitor on Mac.