diff --git a/PyTeX/build/build/builder.py b/PyTeX/build/build/builder.py
index 24c3798..def7bad 100644
--- a/PyTeX/build/build/builder.py
+++ b/PyTeX/build/build/builder.py
@@ -1,5 +1,5 @@
 from pathlib import Path
-from typing import Optional, Union, List, Tuple
+from typing import Optional, Union, List, Tuple, Set
 
 from .enums import PyTeXRootDirType
 from .pytex_config import PyTeXConfig
@@ -9,6 +9,7 @@ from ..versioning.version_info.version_info import VersionInfo, FileVersionInfo
 from .pytex_file import PyTeXSourceFile
 from .relative_path import RelativePath
 from .hashing import md5
+from .pytex_output_file import PyTeXOutputFile
 
 
 class PyTeXBuilder:
@@ -26,17 +27,19 @@ class PyTeXBuilder:
             if isinstance(pytex_config, Path):
                 self._root_dir = pytex_config.parent
             else:
-                self._root_dir: Path = Path('.')  # Working directory
+                self._root_dir = Path('.')  # Working directory
         else:
             self._root_dir = root_dir
 
         # Non-public attributes
         self._build_target_type: Optional[PyTeXRootDirType] = None
         self._old_version_info: Optional[VersionInfo] = None
-        self._modified_old_version_info: VersionInfo = VersionInfo()
+        self._output_files: List[PyTeXOutputFile] = []
         self._pytex_files: List[PyTeXSourceFile] = []
-        self._files_to_clean: List[RelativePath] = []
-        self._files_to_overwrite: List[RelativePath] = []
+        self._files_to_clean: Set[RelativePath] = set()
+        self._files_to_overwrite: Set[RelativePath] = set()
+        self._files_to_build: Set[PyTeXSourceFile] = set()
+        self._tmp_dir: Path = self._root_dir / '.pytex'
 
     def build_tex_sources(self) -> bool:
         self._build_target_type = PyTeXRootDirType.TEX_SOURCE
@@ -156,6 +159,19 @@ class PyTeXBuilder:
         else:
             return FileVersionInfo()
 
+    def _pytex_file_lookup(self, name: str) -> PyTeXSourceFile:
+        matches = [
+            source_file
+            for source_file in self._pytex_files
+            if source_file.relative_path.path.name == name
+        ]
+        if len(matches) >= 2:
+            raise NotImplementedError
+        elif len(matches) == 1:
+            return matches[0]
+        else:
+            raise NotImplementedError  # what to do in this case? dependency does not exist...
+
     def _check_output_directory_integrity(self):
         out_dir_files = [
             RelativePath(self.target_root, file)
@@ -168,25 +184,58 @@ class PyTeXBuilder:
                     raise NotImplementedError  # Not ok
                 else:
                     if self.pytex_config.overwrite_existing_files:
-                        self._files_to_overwrite.append(file)
+                        self._files_to_overwrite.add(file)
                     else:
-                        pass
-                        # Not ok iff we are going to write this file
+                        if file.relative_path in \
+                                {x.relative_path.relative_path for x in self._files_to_build}:
+                            raise NotImplementedError
+                            # Not ok iff we are going to write this file
 
-    def _update_old_version_info(self):
-        self._modified_old_version_info.files = []  # Make sure this is set so we can get references on it
+    def _dependencies_hash(self, pytex_output_file: PyTeXOutputFile) -> str:
+        t = pytex_output_file.dependencies
+        deps: Set[str] = set(t)
+        deps.add(pytex_output_file.source_file.relative_path.path.name)
+        hashes = set()
+        for dep in deps:
+            hashes.add(
+                self._pytex_file_lookup(dep).file_hash
+            )
+        return md5(hashes)
+
+    def _init_output_files(self):
         for source_file in self._pytex_files:
             for output_file in source_file.output_files:
-                self._modified_old_version_info.files.append(
-                    self._old_version_lookup(
-                        output_file
+                self._output_files.append(
+                    PyTeXOutputFile(
+                        output_file=output_file,
+                        source_file=source_file,
+                        last_version_info=self._old_version_lookup(output_file)
                     )
                 )
 
+    def _compute_files_to_build(self):
+        self._files_to_build = {
+            output_file.source_file for output_file in self._output_files
+            if self._dependencies_hash(output_file) != output_file.last_version_info.sources_hash
+            or output_file.last_version_info.file_hash != output_file.file_hash
+        }
+
+    def _build_files(self):
+        for source_file in self._files_to_build:
+            import os
+            out_dir = self._tmp_dir / source_file.file_hash
+            out_dir.mkdir(exist_ok=False, parents=True)
+            source_file.format(self._tmp_dir / source_file.file_hash)
+
     def _build(self) -> bool:
         logger.info("Starting build")
         self._load_pytex_files()
         logger.info(f"Found {len(self._pytex_files)} source files")
-        self._update_old_version_info()
+        self._init_output_files()
+        logger.info(f"Found {len(self._output_files)} potential files to build.")
+        self._compute_files_to_build()
+        logger.info(f"Needing to build {len(self._files_to_build)} many files.")
+        self._check_output_directory_integrity()
+        logger.info(f"Starting build")
+        self._build_files()
         return True
-