diff --git a/radicale/__init__.py b/radicale/__init__.py
index 773d590..8b88d72 100644
--- a/radicale/__init__.py
+++ b/radicale/__init__.py
@@ -200,22 +200,22 @@ class Application:
                 path = item.collection.path
             if self.authorized(user, path, "r"):
                 self.logger.debug(
-                    "%s has read access to collection %s" %
-                    (user or "Anonymous", path or "/"))
+                    "%s has read access to collection %s",
+                    user or "Anonymous", path or "/")
                 read_allowed_items.append(item)
             else:
                 self.logger.debug(
-                    "%s has NO read access to collection %s" %
-                    (user or "Anonymous", path or "/"))
+                    "%s has NO read access to collection %s",
+                    user or "Anonymous", path or "/")
             if self.authorized(user, path, "w"):
                 self.logger.debug(
-                    "%s has write access to collection %s" %
-                    (user or "Anonymous", path or "/"))
+                    "%s has write access to collection %s",
+                    user or "Anonymous", path or "/")
                 write_allowed_items.append(item)
             else:
                 self.logger.debug(
-                    "%s has NO write access to collection %s" %
-                    (user or "Anonymous", path or "/"))
+                    "%s has NO write access to collection %s",
+                    user or "Anonymous", path or "/")
         return read_allowed_items, write_allowed_items
 
     def __call__(self, environ, start_response):
@@ -225,16 +225,16 @@ class Application:
             # Start response
             status = "%i %s" % (
                 status, client.responses.get(status, "Unknown"))
-            self.logger.debug("Answer status: %s" % status)
+            self.logger.debug("Answer status: %s", status)
             start_response(status, list(headers.items()))
             # Return response content
             return [answer] if answer else []
 
         self.logger.debug("\n")  # Add empty lines between requests in debug
-        self.logger.info("%s request at %s received" % (
-            environ["REQUEST_METHOD"], environ["PATH_INFO"]))
+        self.logger.info("%s request at %s received",
+                         environ["REQUEST_METHOD"], environ["PATH_INFO"])
         headers = pprint.pformat(self.headers_log(environ))
-        self.logger.debug("Request headers:\n%s" % headers)
+        self.logger.debug("Request headers:\n%s", headers)
 
         # Strip base_prefix from request URI
         base_prefix = self.configuration.get("server", "base_prefix")
@@ -323,7 +323,7 @@ class Application:
 
         # Set content length
         if answer:
-            self.logger.debug("Response content:\n%s" % answer)
+            self.logger.debug("Response content:\n%s", answer)
             answer = answer.encode(self.encoding)
             accept_encoding = [
                 encoding.strip() for encoding in
@@ -381,7 +381,7 @@ class Application:
         if content_length > 0:
             content = self.decode(
                 environ["wsgi.input"].read(content_length), environ)
-            self.logger.debug("Request content:\n%s" % content.strip())
+            self.logger.debug("Request content:\n%s", content.strip())
         else:
             content = None
         return content
diff --git a/radicale/__main__.py b/radicale/__main__.py
index f319226..c76acca 100644
--- a/radicale/__main__.py
+++ b/radicale/__main__.py
@@ -150,7 +150,7 @@ def serve(configuration, logger):
     logger.info("Starting Radicale")
 
     logger.debug(
-        "Base URL prefix: %s" % configuration.get("server", "base_prefix"))
+        "Base URL prefix: %s", configuration.get("server", "base_prefix"))
 
     # Create collection servers
     servers = {}
@@ -187,8 +187,8 @@ def serve(configuration, logger):
         server = make_server(
             address, port, application, server_class, RequestHandler)
         servers[server.socket] = server
-        logger.debug("Listening to %s port %s" % (
-            server.server_name, server.server_port))
+        logger.debug("Listening to %s port %s",
+                     server.server_name, server.server_port)
         if configuration.getboolean("server", "ssl"):
             logger.debug("Using SSL")
 
diff --git a/radicale/auth.py b/radicale/auth.py
index a9696c2..df3d274 100644
--- a/radicale/auth.py
+++ b/radicale/auth.py
@@ -63,7 +63,7 @@ from importlib import import_module
 def load(configuration, logger):
     """Load the authentication manager chosen in configuration."""
     auth_type = configuration.get("auth", "type")
-    logger.debug("Authentication type is %s" % auth_type)
+    logger.debug("Authentication type is %s", auth_type)
     if auth_type == "None":
         return lambda user, password: True
     elif auth_type == "htpasswd":
diff --git a/radicale/rights.py b/radicale/rights.py
index c2c0fd9..bad50a9 100644
--- a/radicale/rights.py
+++ b/radicale/rights.py
@@ -119,27 +119,27 @@ class Rights(BaseRights):
         regex = ConfigParser(
             {"login": user_escaped, "path": sane_path_escaped})
         if self.rights_type in DEFINED_RIGHTS:
-            self.logger.debug("Rights type '%s'" % self.rights_type)
+            self.logger.debug("Rights type '%s'", self.rights_type)
             regex.readfp(StringIO(DEFINED_RIGHTS[self.rights_type]))
         else:
-            self.logger.debug("Reading rights from file '%s'" % self.filename)
+            self.logger.debug("Reading rights from file '%s'", self.filename)
             if not regex.read(self.filename):
                 self.logger.error(
-                    "File '%s' not found for rights" % self.filename)
+                    "File '%s' not found for rights", self.filename)
                 return False
 
         for section in regex.sections():
             re_user = regex.get(section, "user")
             re_collection = regex.get(section, "collection")
             self.logger.debug(
-                "Test if '%s:%s' matches against '%s:%s' from section '%s'" % (
-                    user, sane_path, re_user, re_collection, section))
+                "Test if '%s:%s' matches against '%s:%s' from section '%s'",
+                user, sane_path, re_user, re_collection, section)
             user_match = re.fullmatch(re_user, user)
             if user_match:
                 re_collection = re_collection.format(*user_match.groups())
                 if re.fullmatch(re_collection, sane_path):
-                    self.logger.debug("Section '%s' matches" % section)
+                    self.logger.debug("Section '%s' matches", section)
                     return permission in regex.get(section, "permission")
                 else:
-                    self.logger.debug("Section '%s' does not match" % section)
+                    self.logger.debug("Section '%s' does not match", section)
         return False