diff --git a/src/Extension/ExtensionInterface.php b/src/Extension/ExtensionInterface.php
index 50752f5d..72e358ee 100644
--- a/src/Extension/ExtensionInterface.php
+++ b/src/Extension/ExtensionInterface.php
@@ -2,20 +2,82 @@
 
 namespace Smarty\Extension;
 
+use Smarty\BlockHandler\BlockHandlerInterface;
+use Smarty\Compile\CompilerInterface;
+use Smarty\Compile\Modifier\ModifierCompilerInterface;
+use Smarty\FunctionHandler\FunctionHandlerInterface;
+
 interface ExtensionInterface {
 
-	public function getTagCompiler(string $tag): ?\Smarty\Compile\CompilerInterface;
+	/**
+	 * Either return \Smarty\Compile\CompilerInterface that will compile the given $tag or
+	 * return null to indicate that you do not know how to handle this $tag. (Another Extension might.)
+	 *
+	 * @param string $tag
+	 * @return CompilerInterface|null
+	 */
+	public function getTagCompiler(string $tag): ?CompilerInterface;
 
-	public function getModifierCompiler(string $modifier): ?\Smarty\Compile\Modifier\ModifierCompilerInterface;
+	/**
+	 * Either return \Smarty\Compile\Modifier\ModifierCompilerInterface that will compile the given $modifier or
+	 * return null to indicate that you do not know how to handle this $modifier. (Another Extension might.)
+	 *
+	 * @param string $modifier
+	 * @return ModifierCompilerInterface|null
+	 */
+	public function getModifierCompiler(string $modifier): ?ModifierCompilerInterface;
 
-	public function getFunctionHandler(string $functionName): ?\Smarty\FunctionHandler\FunctionHandlerInterface;
+	/**
+	 * Either return \Smarty\FunctionHandler\FunctionHandlerInterface that will handle the given $functionName or
+	 * return null to indicate that you do not know how to handle this $functionName. (Another Extension might.)
+	 *
+	 * @param string $functionName
+	 * @return FunctionHandlerInterface|null
+	 */
+	public function getFunctionHandler(string $functionName): ?FunctionHandlerInterface;
 
-	public function getBlockHandler(string $blockTagName): ?\Smarty\BlockHandler\BlockHandlerInterface;
+	/**
+	 * Either return \Smarty\BlockHandler\BlockHandlerInterface that will handle the given $blockTagName or return null
+	 * to indicate that you do not know how to handle this $blockTagName. (Another Extension might.)
+	 *
+	 * @param string $blockTagName
+	 * @return BlockHandlerInterface|null
+	 */
+	public function getBlockHandler(string $blockTagName): ?BlockHandlerInterface;
 
+	/**
+	 * Either return a callable that takes at least 1 parameter (a string) and returns a modified string or return null
+	 * to indicate that you do not know how to handle this $modifierName. (Another Extension might.)
+	 *
+	 * The callable can accept additional optional parameters.
+	 *
+	 * @param string $modifierName
+	 * @return callable|null
+	 */
 	public function getModifierCallback(string $modifierName);
 
+	/**
+	 * Return a list of prefilters that will all be applied, in sequence.
+	 * Template prefilters can be used to preprocess templates before they are compiled.
+	 *
+	 * @return \Smarty\Filter\FilterInterface[]
+	 */
 	public function getPreFilters(): array;
+
+	/**
+	 * Return a list of postfilters that will all be applied, in sequence.
+	 * Template postfilters can be used to process compiled template code (so, after the compilation).
+	 *
+	 * @return \Smarty\Filter\FilterInterface[]
+	 */
 	public function getPostFilters(): array;
+
+	/**
+	 * Return a list of outputfilters that will all be applied, in sequence.
+	 * Template outputfilters can be used to change template output just before it is rendered.
+	 *
+	 * @return \Smarty\Filter\FilterInterface[]
+	 */
 	public function getOutputFilters(): array;
 
 }
\ No newline at end of file