diff --git a/src/python/jw/pkg/App.py b/src/python/jw/pkg/App.py
index a9ab6eb6..af9b9311 100644
--- a/src/python/jw/pkg/App.py
+++ b/src/python/jw/pkg/App.py
@@ -98,6 +98,29 @@ class App(Base):
             return None
         raise Exception('No project path found for module "{}"'.format(name))
 
+    @cache
+    def __is_installed(self, name: str, devel: bool) -> bool:
+        # devel: the project is in the dev tree or the -devel
+        # package is installed: make/project.conf is present.
+        # run: the -run package is installed: a VERSION file is
+        # present in the project directory or
+        # /usr/share/doc/packages.
+        if devel and name == self.__top_name:
+            # the topdir is the project's own buildable checkout
+            if os.path.exists(f'{self.__topdir}/make/project.conf'):
+                return True
+        search, file = (
+            (self.__projs_root, self.___opt_root),
+            '/make/project.conf'
+        ) if devel else (
+            (self.__projs_root, '/usr/share/doc/packages'),
+            '/VERSION'
+        )
+        for root in search:
+            if root is not None and os.path.exists(f'{root}/{name}{file}'):
+                return True
+        return False
+
     def __find_dir(
         self,
         name: str,
@@ -187,6 +210,7 @@ class App(Base):
         names_only: bool,
     ) -> None:
         name = self.strip_module_from_spec(spec)
+        mod = re.split('([=><]+)', spec)[0].strip()
         if names_only:
             spec = name
         if spec in buf:
@@ -196,6 +220,15 @@ class App(Base):
                 buf.append(spec)
             return
         visited.add(spec)
+        needed_subpackage = 'devel' if mod.endswith(('-dev', '-devel')) else 'run'
+        if not self.is_installed(name, devel = True):
+            if not mod.endswith(('-dev', '-devel')):
+                if self.is_installed(name, devel = False):
+                    return
+            raise Exception(
+                f'Unmet dependency on {mod}: the -{needed_subpackage} package '
+                f'of project {name} is not installed'
+            )
         vals = self.get_value(name, section, key)
         log(
             DEBUG,
@@ -404,6 +437,18 @@ class App(Base):
             raise Exception('No distro object')
         return self.__distro
 
+    def is_installed(
+        self,
+        name: str,
+        devel: bool = False,
+    ) -> bool:
+        """True if the project is installed: for devel,
+        make/project.conf is present in the dev tree or /opt; for
+        run, a VERSION file is present in the project directory
+        or /usr/share/doc/packages.
+        """
+        return self.__is_installed(name, devel)
+
     def find_dir(
         self,
         name: str,