diff --git a/.env.example b/.env.example
index 4962895..dc2fe20 100644
--- a/.env.example
+++ b/.env.example
@@ -26,3 +26,13 @@ DASH_DEBUG=true
 
 MOCK_PLATFORM_HEADER=82
 MOCK_PLATFORM_SPACER=20
+
+
+# ---------------------------------------------------------------------------
+# Development aid: print memory usage to the terminal
+# ---------------------------------------------------------------------------
+# Prints the app's RSS memory usage every time you click a page link.
+#
+# Off by default. Needs psutil, which is only in requirements-dev.txt, so
+# never turn this on anywhere requirements.txt alone was installed.
+LOG_MEMORY=false
diff --git a/README.md b/README.md
index 7bf68eb..6a2119e 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,12 @@ properties on the page root, and `.shell` pads itself by those same
 variables — that's what actually reserves the space so page content doesn't
 render underneath the bars.
 
+## Memory logging
+
+Set `LOG_MEMORY=true` in `.env` to print the app's RSS memory usage to the
+terminal every time you click a page link. It's a dev aid only, off by
+default.
+
 ## Styling / theming
 
 Colours, fonts and spacing live in two places that must be kept in step:
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 3568291..8627e39 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -6,3 +6,4 @@
 pytest>=8.0
 black>=24.0
 pre-commit>=3.7
+psutil>=5.9
diff --git a/src/app.py b/src/app.py
index 7662bf1..26b472e 100644
--- a/src/app.py
+++ b/src/app.py
@@ -14,6 +14,7 @@
 from dash import Dash, Input, Output, callback, dcc, html
 from dotenv import load_dotenv
 
+import memory_log
 import theme  # registers the "4subsea" Plotly template
 
 PAGE_TITLE = "Dashboard Template"
@@ -25,6 +26,7 @@
 DASH_DEBUG = os.getenv("DASH_DEBUG", "true").strip().lower() == "true"
 MOCK_PLATFORM_HEADER = int(os.getenv("MOCK_PLATFORM_HEADER", "0") or 0)
 MOCK_PLATFORM_SPACER = int(os.getenv("MOCK_PLATFORM_SPACER", "20") or 20)
+LOG_MEMORY = os.getenv("LOG_MEMORY", "false").strip().lower() == "true"
 
 app = Dash(
     __name__,
@@ -126,6 +128,8 @@ def mock_4insight():
 @callback(Output("nav-bar", "children"), Input("url", "pathname"))
 def highlight_active_tab(pathname):
     """Function to highlight the active tab in the sidebar based on the current URL path."""
+    if LOG_MEMORY:
+        memory_log.log_once(pathname)
     return nav_bar(pathname)
 
 
diff --git a/src/memory_log.py b/src/memory_log.py
new file mode 100644
index 0000000..be9d4a7
--- /dev/null
+++ b/src/memory_log.py
@@ -0,0 +1,23 @@
+"""Prints RSS memory usage to the terminal on demand.
+
+Opt-in dev aid - see LOG_MEMORY in app.py and .env.example. Nothing here
+writes to a file or feeds a log collector; it is only for watching a local
+`python src/app.py` session by eye.
+"""
+
+_process = None
+
+
+def _rss_mb():
+    global _process
+    import psutil
+
+    if _process is None:
+        _process = psutil.Process()
+    return _process.memory_info().rss / (1024 * 1024)
+
+
+def log_once(note=""):
+    """Print one line right now. `note` names what triggered it, e.g. a path."""
+    suffix = f" ({note})" if note else ""
+    print(f"[memory] {_rss_mb():.1f} MB RSS{suffix}")
diff --git a/tests/test_app.py b/tests/test_app.py
index 3772939..8dd30a8 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -150,3 +150,24 @@ def test_the_mock_header_carries_the_logo_and_placeholder_title_when_configured(
     assert bar.style["height"] == "82px"
     assert bottom_bar.style["height"] == "20px"
     assert text_of(bottom_bar.children) == "simulated spacer — not part of the app"
+
+
+# ---------------------------------------------------------------------------
+# Memory logging
+# ---------------------------------------------------------------------------
+
+
+def test_navigation_logs_memory_when_enabled(monkeypatch):
+    monkeypatch.setattr(app, "LOG_MEMORY", True)
+    calls = []
+    monkeypatch.setattr(app.memory_log, "log_once", lambda note="": calls.append(note))
+    app.highlight_active_tab("/analytics")
+    assert calls == ["/analytics"]
+
+
+def test_navigation_does_not_log_memory_when_disabled(monkeypatch):
+    monkeypatch.setattr(app, "LOG_MEMORY", False)
+    calls = []
+    monkeypatch.setattr(app.memory_log, "log_once", lambda note="": calls.append(note))
+    app.highlight_active_tab("/analytics")
+    assert calls == []