diff --git a/src/lib.rs b/src/lib.rs
index 215c1fe..2edd454 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -309,6 +309,7 @@ impl Assert {
         self.expect_stdout = Some(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
+            expected_result: true,
             kind: StdOut,
         });
         self
@@ -329,6 +330,7 @@ impl Assert {
         self.expect_stdout = Some(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
+            expected_result: true,
             kind: StdOut,
         });
         self
@@ -351,6 +353,7 @@ impl Assert {
         self.expect_stderr = Some(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
+            expected_result: true,
             kind: StdErr,
         });
         self
@@ -373,6 +376,99 @@ impl Assert {
         self.expect_stderr = Some(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
+            expected_result: true,
+            kind: StdErr,
+        });
+        self
+    }
+
+    /// Expect the command's output to not **contain** `output`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// extern crate assert_cli;
+    ///
+    /// assert_cli::Assert::command(&["echo", "42"])
+    ///     .doesnt_print("73")
+    ///     .execute()
+    ///     .unwrap();
+    /// ```
+    pub fn doesnt_print<O: Into<String>>(mut self, output: O) -> Self {
+        self.expect_stdout = Some(OutputAssertion {
+            expect: output.into(),
+            fuzzy: true,
+            expected_result: false,
+            kind: StdOut,
+        });
+        self
+    }
+
+    /// Expect the command to output to not be **exactly** this `output`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// extern crate assert_cli;
+    ///
+    /// assert_cli::Assert::command(&["echo", "42"])
+    ///     .doesnt_print_exactly("73")
+    ///     .execute()
+    ///     .unwrap();
+    /// ```
+    pub fn doesnt_print_exactly<O: Into<String>>(mut self, output: O) -> Self {
+        self.expect_stdout = Some(OutputAssertion {
+            expect: output.into(),
+            fuzzy: false,
+            expected_result: false,
+            kind: StdOut,
+        });
+        self
+    }
+
+    /// Expect the command's stderr output to not **contain** `output`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// extern crate assert_cli;
+    ///
+    /// assert_cli::Assert::command(&["cat", "non-existing-file"])
+    ///     .fails()
+    ///     .and()
+    ///     .doesnt_print_error("content")
+    ///     .execute()
+    ///     .unwrap();
+    /// ```
+    pub fn doesnt_print_error<O: Into<String>>(mut self, output: O) -> Self {
+        self.expect_stderr = Some(OutputAssertion {
+            expect: output.into(),
+            fuzzy: true,
+            expected_result: false,
+            kind: StdErr,
+        });
+        self
+    }
+
+    /// Expect the command to output to not be **exactly** this `output` to stderr.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// extern crate assert_cli;
+    ///
+    /// assert_cli::Assert::command(&["cat", "non-existing-file"])
+    ///     .fails_with(1)
+    ///     .and()
+    ///     .doesnt_print_error_exactly("content")
+    ///     .execute()
+    ///     .unwrap();
+    /// ```
+    pub fn doesnt_print_error_exactly<O: Into<String>>(mut self, output: O) -> Self {
+        self.expect_stderr = Some(OutputAssertion {
+            expect: output.into(),
+            fuzzy: false,
+            expected_result: false,
             kind: StdErr,
         });
         self
diff --git a/src/output.rs b/src/output.rs
index 34c975c..b95fee2 100644
--- a/src/output.rs
+++ b/src/output.rs
@@ -11,13 +11,19 @@ use diff;
 pub struct OutputAssertion<T> {
     pub expect: String,
     pub fuzzy: bool,
+    pub expected_result: bool,
     pub kind: T,
 }
 
 impl<T: OutputType> OutputAssertion<T> {
     fn matches_fuzzy(&self, got: &str) -> Result<()> {
-        if !got.contains(&self.expect) {
-            bail!(ErrorKind::OutputMismatch(self.expect.clone(), got.into()));
+        let result = got.contains(&self.expect);
+        if result != self.expected_result {
+            if self.expected_result {
+                bail!(ErrorKind::OutputDoesntContain(self.expect.clone(), got.into()));
+            } else {
+                bail!(ErrorKind::OutputContains(self.expect.clone(), got.into()));
+            }
         }
 
         Ok(())
@@ -25,10 +31,15 @@ impl<T: OutputType> OutputAssertion<T> {
 
     fn matches_exact(&self, got: &str) -> Result<()> {
         let differences = Changeset::new(self.expect.trim(), got.trim(), "\n");
-
-        if differences.distance > 0 {
-            let nice_diff = diff::render(&differences)?;
-            bail!(ErrorKind::ExactOutputMismatch(nice_diff));
+        let result = differences.distance == 0;
+
+        if result != self.expected_result {
+            if self.expected_result {
+                let nice_diff = diff::render(&differences)?;
+                bail!(ErrorKind::OutputDoesntMatch(nice_diff));
+            } else {
+                bail!(ErrorKind::OutputMatches(got.to_owned()));
+            }
         }
 
         Ok(())
@@ -88,14 +99,22 @@ mod errors {
             Fmt(::std::fmt::Error);
         }
         errors {
-            OutputMismatch(expected: String, got: String) {
+            OutputDoesntContain(expected: String, got: String) {
                 description("Output was not as expected")
                 display("expected to contain {:?}, got {:?}", expected, got)
             }
-            ExactOutputMismatch(diff: String) {
+            OutputContains(expected: String, got: String) {
+                description("Output was not as expected")
+                display("expected to not contain {:?}, got {:?}", expected, got)
+            }
+            OutputDoesntMatch(diff: String) {
                 description("Output was not as expected")
                 display("{}", diff)
             }
+            OutputMatches(got: String) {
+                description("Output was not as expected")
+                display("{}", got)
+            }
         }
     }
 }