diff --git a/src/Command.php b/src/Command.php
index cf39eb3..06b0783 100644
--- a/src/Command.php
+++ b/src/Command.php
@@ -341,6 +341,36 @@ public function validate(array $arguments, array $options) : array
         );
     }
 
+    /**
+     * Apply definition defaults to the console.
+     *
+     * Any defined argument or option that was not passed on the command line
+     * and declares a "default" key gets its default written back into the
+     * Console, so getArgument() and getOption() return the resolved value.
+     * Values are stored as strings (or booleans for flags) because the
+     * Console accessors are typed that way.
+     *
+     * @param Console $console The console the command is running on
+     */
+    public function applyDefaults(Console $console) : void
+    {
+        $arguments = $console->getArguments();
+        foreach ($this->argumentDefinitions as $key => $definition) {
+            $default = $definition['default'] ?? null;
+            if (!\array_key_exists($key, $arguments) && \is_scalar($default)) {
+                $console->setArgument((int) $key, (string) $default);
+            }
+        }
+        $options = $console->getOptions();
+        foreach ($this->optionDefinitions as $key => $definition) {
+            $default = $definition['default'] ?? null;
+            if (\array_key_exists($key, $options) || !\is_scalar($default)) {
+                continue;
+            }
+            $console->setOption((string) $key, \is_bool($default) ? $default : (string) $default);
+        }
+    }
+
     /**
      * Validate a set of values against their definitions.
      *
diff --git a/src/Console.php b/src/Console.php
index a5c2b24..8323604 100644
--- a/src/Console.php
+++ b/src/Console.php
@@ -140,6 +140,38 @@ public function getArgument(int $position) : ?string
         return $this->arguments[$position] ?? null;
     }
 
+    /**
+     * Set an argument value.
+     *
+     * Used by Command::applyDefaults() to fill in definition defaults.
+     *
+     * @param int $position Argument position, starting from zero
+     * @param string $value The argument value
+     *
+     * @return static
+     */
+    public function setArgument(int $position, string $value) : static
+    {
+        $this->arguments[$position] = $value;
+        return $this;
+    }
+
+    /**
+     * Set an option value.
+     *
+     * Used by Command::applyDefaults() to fill in definition defaults.
+     *
+     * @param string $name The option name
+     * @param bool|string $value The option value
+     *
+     * @return static
+     */
+    public function setOption(string $name, bool | string $value) : static
+    {
+        $this->options[$name] = $value;
+        return $this;
+    }
+
     /**
      * Set the Language instance.
      *
@@ -324,6 +356,7 @@ protected function dispatch() : void
             $this->validationFailed($errors);
             return;
         }
+        $command->applyDefaults($this);
         $command->run();
     }
 
diff --git a/tests/ValidationTest.php b/tests/ValidationTest.php
index c31fb5f..b3acbc4 100644
--- a/tests/ValidationTest.php
+++ b/tests/ValidationTest.php
@@ -173,4 +173,59 @@ public function testGettersReturnTheDefinitions() : void
         self::assertSame([0 => ['type' => 'int']], $command->getArgumentDefinitions());
         self::assertSame(['count' => ['type' => 'int']], $command->getOptionDefinitions());
     }
+
+    public function testDefaultArgumentIsAppliedWhenMissing() : void
+    {
+        $command = new ValidatedCommandMock($this->console);
+        $command->setArgumentDefinitions([
+            0 => ['type' => 'int', 'default' => '7'],
+        ]);
+        $this->console->addCommand($command);
+        $this->console->exec('validated');
+        self::assertSame('7', $this->console->getArgument(0));
+        self::assertStringContainsString('ran', Stdout::getContents());
+    }
+
+    public function testDefaultOptionIsAppliedWhenMissing() : void
+    {
+        $command = new ValidatedCommandMock($this->console);
+        $command->setOptionDefinitions([
+            'count' => ['type' => 'int', 'default' => '3'],
+            'verbose' => ['type' => 'flag', 'default' => true],
+        ]);
+        $this->console->addCommand($command);
+        $this->console->exec('validated');
+        self::assertSame('3', $this->console->getOption('count'));
+        self::assertTrue($this->console->getOption('verbose'));
+        self::assertStringContainsString('ran', Stdout::getContents());
+    }
+
+    public function testPassedValuesAreNotOverriddenByDefaults() : void
+    {
+        $command = new ValidatedCommandMock($this->console);
+        $command->setArgumentDefinitions([
+            0 => ['type' => 'int', 'default' => '7'],
+        ]);
+        $command->setOptionDefinitions([
+            'count' => ['type' => 'int', 'default' => '3'],
+        ]);
+        $this->console->addCommand($command);
+        $this->console->exec('validated 42 --count=5');
+        self::assertSame('42', $this->console->getArgument(0));
+        self::assertSame('5', $this->console->getOption('count'));
+    }
+
+    public function testApplyDefaultsDirectlyFillsTheConsole() : void
+    {
+        $command = new ValidatedCommandMock($this->console);
+        $command->setArgumentDefinitions([
+            0 => ['default' => 'fallback'],
+        ]);
+        $command->setOptionDefinitions([
+            'name' => ['default' => 'cli'],
+        ]);
+        $command->applyDefaults($this->console);
+        self::assertSame('fallback', $this->console->getArgument(0));
+        self::assertSame('cli', $this->console->getOption('name'));
+    }
 }