diff --git a/Makefile b/Makefile
index dcf383a4..901e0ff8 100644
--- a/Makefile
+++ b/Makefile
@@ -5,19 +5,28 @@
 # from the environment for the first two.
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
+SPHINXDIR     = .sphinx
 SOURCEDIR     = .
 BUILDDIR      = _build
-VENV          = .sphinx/venv/bin/activate
+VENVDIR       = $(SPHINXDIR)/venv
+VENV          = $(VENVDIR)/bin/activate
 
 
 # Put it first so that "make" without argument is like "make help".
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
-install:
+.PHONY: help
+
+# Explicit target avoids fall-through to the "Makefile" target.
+$(SPHINXDIR)/requirements.txt:
+	test -f $(SPHINXDIR)/requirements.txt
+
+# If requirements are updated, venv should be rebuilt and timestamped.
+$(VENVDIR): $(SPHINXDIR)/requirements.txt
 	@echo "... setting up virtualenv"
-	python3 -m venv .sphinx/venv
-	. $(VENV); pip install --upgrade -r .sphinx/requirements.txt
+	python3 -m venv $(VENVDIR)
+	. $(VENV); pip install --upgrade -r $(SPHINXDIR)/requirements.txt
 
 	@echo "\n" \
 		"--------------------------------------------------------------- \n" \
@@ -26,40 +35,77 @@ install:
                 "* only serve: make serve \n" \
                 "* clean built doc files: make clean-doc \n" \
                 "* clean full environment: make clean \n" \
-		"* check spelling: make spelling \n" \
+                "* check spelling: make spelling \n" \
                 "* check inclusive language: make woke \n" \
+                "* other possible targets: make <press TAB twice> \n" \
 		"--------------------------------------------------------------- \n"
-run:
+	@touch $(VENVDIR)
+
+
+install: $(VENVDIR)
+
+.PHONY:  install
+
+
+run: install
 	. $(VENV); sphinx-autobuild -c . -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)"
 
-html:
+.PHONY: run
+
+# Doesn't depend on $(BUILDDIR) to rebuild properly at every run.
+html: install
 	. $(VENV); $(SPHINXBUILD) -c . -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)" -w .sphinx/warnings.txt
 
-epub:
+.PHONY: html
+
+
+epub: install
 	. $(VENV); $(SPHINXBUILD) -c . -b epub "$(SOURCEDIR)" "$(BUILDDIR)" -w .sphinx/warnings.txt
 
-serve:
+.PHONY: epub
+
+
+serve: html
 	cd "$(BUILDDIR)"; python3 -m http.server 8000
 
+.PHONY: serve
+
+
 clean: clean-doc
-	rm -rf .sphinx/venv
+	@test ! -e "$(VENVDIR)" -o -d "$(VENVDIR)" -a "$(abspath $(VENVDIR))" != "$(VENVDIR)"
+	rm -rf $(VENVDIR)
+
+.PHONY: clean
+
 
 clean-doc:
 	git clean -fx "$(BUILDDIR)"
 
+.PHONY: clean-doc
+
+
 spelling: html
 	. $(VENV) ; python3 -m pyspelling -c .sphinx/spellingcheck.yaml
 
-linkcheck:
+.PHONY: spelling
+
+
+linkcheck: install
 	. $(VENV) ; $(SPHINXBUILD) -c . -b linkcheck  "$(SOURCEDIR)" "$(BUILDDIR)"
 
+.PHONY: linkcheck
+
+
 woke:
 	type woke >/dev/null 2>&1 || { sudo snap install woke; }
 	woke *.rst **/*.rst -c https://github.com/canonical-web-and-design/Inclusive-naming/raw/main/config.yml
 
-.PHONY: help Makefile
+.PHONY: woke
+
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	. $(VENV); $(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: Makefile