diff --git a/.gitignore b/.gitignore
index f1d8d08..b273408 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 *.log
 **/__pycache__
 *~
+index.json
diff --git a/__main__.py b/__main__.py
index 68d18c7..d286bac 100644
--- a/__main__.py
+++ b/__main__.py
@@ -71,11 +71,14 @@ if __name__ == "__main__":
     logging.info("Beginning parsing")
         
     threads = []
+    comics = []
     thread_limit = threading.BoundedSemaphore(value=args.threads)
     
     for url in args.url:
         comic_type = find_comic_class(url.netloc + url.path)
         comic = comic_type.create_from_url(url)
+        comics.append(comic)
+        comic.read_index(base_path)
         comic.await_load()
         logging.info("There are %d comics", comic.latest_identifier)
         if args.latest:
@@ -85,11 +88,19 @@ if __name__ == "__main__":
             start = args.start or comic.get_first_strip()
             end = args.end or comic.get_latest_strip()
             r = r[start - 1:end]
-    
-        for index in reversed(r):
-            t = threading.Thread(name=str(comic.get_identifier_string()) + "-" + str(index), target=load_strip, args=(base_path, comic, index, args.plain, thread_limit))
-            threads.append(t)
-            t.start()
 
-    for t in threads:
-        t.join()
+        try:
+            for index in reversed(r):
+                if not comic.is_strip_present(base_path, ".cbz", str(index)):
+                    t = threading.Thread(name=str(comic.get_identifier_string()) + "-" + str(index), target=load_strip, args=(base_path, comic, index, args.plain, thread_limit))
+                    threads.append(t)
+                    t.start()
+        finally:
+            comic.save_index(base_path)
+
+    try:
+        for t in threads:
+            t.join()
+    finally:
+        for comic in comics:
+            comic.save_index(base_path)
diff --git a/comic_download/comic_strip.py b/comic_download/comic_strip.py
index dd7c42f..65d6c11 100644
--- a/comic_download/comic_strip.py
+++ b/comic_download/comic_strip.py
@@ -263,6 +263,73 @@ class Comic(ImageRepo, ABC):
         """The title of the comic."""
         self.author = None
         """The author(s) of the comic. Can be a string for a name, a list of names, or a list of objects containing the names under "name" and role under "role"."""
+        
+        self.strip_index = {}
+        """
+        The strip index maps comic identifiers to their filenames (sans stems). If we
+        cache this data, we can find out where each strip lives without having to
+        load data from the website, thus using less requests for already-downloaded
+        strips.
+        """
+        self._index_loaded = False
+        """A flag for when the index has been successfully loaded."""
+        self._index_lock = Condition(Lock())
+        """A lock for enforcing thread safety when loading the comic index file."""
+
+    def read_index(self, base_path: Path):
+        """
+        Saves the strip index to the same base path we download comic strips to.
+        It is saved to "index.json."
+
+        :param base_path: The folder to save the index to.
+        """
+        with self._index_lock:
+            if not self._index_loaded:
+                index_path = Path(base_path, "index.json")
+                if index_path.exists():
+                    try:
+                        with open(Path(base_path, "index.json"), 'r') as index_fp:
+                            self.strip_index = json.load(index_fp)
+                    except json.JSONDecodeError as e:
+                        logging.exception(e)
+                        self.strip_index = {}
+                self._index_loaded = True
+            self._index_lock.notify_all()
+
+    def save_index(self, base_path: Path):
+        """
+        Saves the strip index to the same base path we download comic strips to.
+        It is saved to "index.json."
+
+        :param base_path: The folder to save the index to.
+        """
+        with self._index_lock:
+            with open(Path(base_path, "index.json"), 'w') as index_fp:
+                json.dump(self.strip_index, index_fp)
+
+    def index_strip(self, strip):
+        """
+        Registers a comic to the index. This does not save the comic index.
+
+        :param strip: The comic strip to index
+        """
+        with self._index_lock:
+            self.strip_index[strip.identifier] = strip.get_filename()
+
+    def is_strip_present(self, base_path: Path, suffix: str, identifier) -> bool:
+        """
+        Checks to see if a given strip has been saved to the file.
+
+        :param base_path: The folder we expect the strip to show up in.
+        :param suffix: The type suffix we want to check (i.e. ".cbz")
+        :param identifier: The identifier to check.
+        :returns: True if the strip file has been saved.
+        """
+        if not identifier in self.strip_index:
+            return False
+        path = Path(base_path, self.strip_index[identifier] + suffix)
+        return path.exists()
+            
 
     #@abstractmethod
     @classmethod
@@ -382,6 +449,7 @@ class ComicStrip(ImageRepo, ABC):
     def load_data(self):
         super().load_data()
         self.comic.load_data()
+        self.comic.index_strip(self)
 
     def download_data(self):
         super().download_data()