diff --git a/practices/doc-comments/.gitignore b/practices/doc-comments/.gitignore
new file mode 100644
index 0000000..96ef6c0
--- /dev/null
+++ b/practices/doc-comments/.gitignore
@@ -0,0 +1,2 @@
+/target
+Cargo.lock
diff --git a/practices/doc-comments/Cargo.lock b/practices/doc-comments/Cargo.lock
new file mode 100644
index 0000000..9477981
--- /dev/null
+++ b/practices/doc-comments/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "doc-comments"
+version = "0.1.0"
diff --git a/practices/doc-comments/Cargo.toml b/practices/doc-comments/Cargo.toml
new file mode 100644
index 0000000..335de20
--- /dev/null
+++ b/practices/doc-comments/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "doc-comments"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/practices/doc-comments/src/compute.rs b/practices/doc-comments/src/compute.rs
new file mode 100644
index 0000000..afc0842
--- /dev/null
+++ b/practices/doc-comments/src/compute.rs
@@ -0,0 +1,36 @@
+//! Do some complicated arithmetic that you can't do by yourself
+
+/// # Panics
+///
+/// The function panics if the second argument is zero.
+///
+/// ```rust,should_panic
+/// // panics on division by zero
+/// doc_comments::compute::div(10, 0);
+/// ```
+pub fn div(a: i32, b: i32) -> i32 {
+    if b == 0 {
+        panic!("Divide-by-zero error");
+    }
+
+    a / b
+}
+
+
+/// ```
+/// # fn try_main() -> Result<(), String> {
+/// let res = doc_comments::compute::try_div(10, 1)?;
+/// # Ok(()) // returning from try_main
+/// # }
+/// # fn main() {
+/// #    try_main().unwrap();
+/// #
+/// # }
+/// ```
+pub fn try_div(a: i32, b: i32) -> Result<i32, String> {
+    if b == 0 {
+        Err(String::from("Divide-by-zero"))
+    } else {
+        Ok(a / b)
+    }
+}
\ No newline at end of file
diff --git a/practices/doc-comments/src/lib.rs b/practices/doc-comments/src/lib.rs
new file mode 100644
index 0000000..645b77f
--- /dev/null
+++ b/practices/doc-comments/src/lib.rs
@@ -0,0 +1,54 @@
+//! # Doc comments
+//! 
+//! A library for showing how to use doc comments
+
+pub mod compute;
+
+/// Add one to the given value and return a new value
+///
+/// # Examples
+///
+/// ```
+/// let arg = 5;
+/// let answer = doc_comments::add_one(arg);
+///
+/// assert_eq!(6, answer);
+/// ```
+pub fn add_one(x: i32) -> i32 {
+    x + 1
+}
+
+
+
+/** Add two to the given value and return a new value
+
+# Examples
+
+```
+let arg = 5;
+let answer = doc_comments::add_two(arg);
+
+assert_eq!(7, answer);
+```
+*/
+pub fn add_two(x: i32) -> i32 {
+    x + 2
+}
+
+
+/// Add three to the given value and return a [`Option`] type
+pub fn add_three(x: i32) -> Option<i32> {
+    Some(x + 3)
+}
+
+mod a {
+    ///  Add four to the given value and return a [`Option`] type
+    /// [`crate::MySpecialFormatter`]
+    pub fn add_four(x: i32) -> Option<i32> {
+        Some(x + 4)
+    }
+}
+
+struct MySpecialFormatter;
+
+
diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 0133fa2..1ac15c1 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -43,8 +43,8 @@
   - [Package and Crate](crate-module/crate.md)
   - [Module](crate-module/module.md)
   - [Advanced use and pub](crate-module/use-pub.md)
-- [Comments and Docs TODO](comments-docs.md)
-- [Formatted output TODO](formatted-output.md)
+- [Comments and Docs](comments-docs.md)
+- [Formatted output](formatted-output.md)
 - [Lifetime TODO](lifetime/intro.md)
   - [basic](lifetime/basic.md)
   - [&'static and T: 'static](lifetime/static.md)
diff --git a/src/comments-docs.md b/src/comments-docs.md
index cb7d70d..3c0894d 100644
--- a/src/comments-docs.md
+++ b/src/comments-docs.md
@@ -1 +1,295 @@
 # Comments and Docs
+Every program requires comments:
+
+
+## Comments
+- Regular comments which are ignored by the compiler:
+  - `// Line comment, which goes to the end of the line`
+  - `/* Block comment, which goes to the end of the closing delimiter */`
+
+
+### Examples
+```rust
+fn main() {
+    // This is an example of a line comment
+    // There are two slashes at the beginning of the line
+    // And nothing written inside these will be read by the compiler
+
+    // println!("Hello, world!");
+
+    // Run it. See? Now try deleting the two slashes, and run it again.
+
+    /* 
+     * This is another type of comment, a block comment. In general,
+     * line comments are the recommended comment style. But
+     * block comments are extremely useful for temporarily disabling
+     * chunks of code. /* Block comments can be /* nested, */ */
+     * so it takes only a few keystrokes to comment out everything
+     * in this main() function. /*/*/* Try it yourself! */*/*/
+     */
+
+    /*
+    Note: The previous column of `*` was entirely for style. There's
+    no actual need for it.
+    */
+}
+```
+
+### Exercises
+1. 🌟🌟
+```rust,editable
+
+/* Make it work, only using comments! */
+fn main() {
+    todo!();
+    unimplemented!();
+
+    assert_eq!(6, 5 + 3 + 2 + 1 )
+}
+```
+
+
+## Doc Comments
+- Doc comments which are parsed into HTML and supported `Markdown`
+  - `/// Generate library docs for the following item`
+  - `//! Generate library docs for the eclosing item`
+
+Before starting, we need to create a new package for practice: `cargo new --lib doc-comments`.
+
+
+### Line doc comments `///` 
+Add docs for function `add_one`
+```rust
+// in lib.rs
+
+/// Add one to the given value and return the value
+///
+/// # Examples
+///
+/// ```
+/// let arg = 5;
+/// let answer = my_crate::add_one(arg);
+///
+/// assert_eq!(6, answer);
+/// ```
+pub fn add_one(x: i32) -> i32 {
+    x + 1
+}
+```
+
+### Cargo doc
+We can use `cargo doc --open` to generate html files and open them in the browser.
+
+### Block doc comments `/** ... */`
+Add docs for function `add_two`:
+```rust
+/** Add two to the given value and return a new value
+
+# Examples
+
+let arg = 5;
+let answer = my_crate::add_two(arg);
+
+assert_eq!(7, answer);
+
+*/
+pub fn add_two(x: i32) -> i32 {
+    x + 2
+}
+```
+
+### Doc comments for crate and module
+We can also add doc comments for our crates and modules.
+
+Firstly, let's add some doc comments for our library crate:
+
+> Note: We mush place crates and module comments at the top of the crate root or module file.
+```rust
+//! # Doc comments
+//! 
+//! A library for showing how to use doc comments
+
+// in lib.rs
+pub mod compute;
+```
+
+You can also use block comments to achieve this:
+```rust
+/*! # Doc comments
+
+ A library for showing how to use doc comments */
+```
+
+Next, create a new module file `src/compute.rs`, and add following comments to it:
+```rust
+//! //! Do some complicated arithmetic that you can't do by yourself
+
+// in compute.rs
+```
+
+Then run `cargo doc --open` and see the results.
+
+
+### Doc tests
+The doc comments of `add_one` and `add_tow` contain two example code blocks.
+
+The examples can not only demonstrate how to use your library, but also running as test with `cargo test` command.
+
+2. 🌟🌟 But there are errors in the two examples, please fix them, and running with `cargo test` to get following result: 
+```shell
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
+
+   Doc-tests doc-comments
+
+running 2 tests
+test src/lib.rs - add_one (line 11) ... ok
+test src/lib.rs - add_two (line 26) ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.55s
+```
+
+3. 🌟🌟 Sometimes we expect an example to be panic, add following code to `src/compute.rs` and make the `cargo test` passed.
+
+> You can only modify the comments, DON'T modify `fn div`
+
+```rust
+// in src/compute.rs
+
+/// # Panics
+///
+/// The function panics if the second argument is zero.
+///
+/// ```rust,should_panic
+/// // panics on division by zero
+/// doc_comments::compute::div(10, 0);
+/// ```
+pub fn div(a: i32, b: i32) -> i32 {
+    if b == 0 {
+        panic!("Divide-by-zero error");
+    }
+
+    a / b
+}
+```
+
+4. 🌟🌟 Sometimes we want to hide the doc comments, but keep the doc tests.
+
+Add following code to `src/compute.rs` ,
+
+```rust
+// in src/compute.rs
+
+/// ```
+/// # fn try_main() -> Result<(), String> {
+/// let res = doc_comments::compute::try_div(10, 0)?;
+/// # Ok(()) // returning from try_main
+/// # }
+/// # fn main() { 
+/// #    try_main().unwrap();
+/// #
+/// # }
+/// ```
+pub fn try_div(a: i32, b: i32) -> Result<i32, String> {
+    if b == 0 {
+        Err(String::from("Divide-by-zero"))
+    } else {
+        Ok(a / b)
+    }
+}
+```
+
+and modify this code to achieve two goals:
+
+- The doc comments must not be presented in html files generated by `cargo doc --open`
+- run the tests, you should see results as below:
+
+```shell
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
+
+   Doc-tests doc-comments
+
+running 4 tests
+test src/compute.rs - compute::div (line 7) ... ok
+test src/lib.rs - add_two (line 27) ... ok
+test src/lib.rs - add_one (line 11) ... ok
+test src/compute.rs - compute::try_div (line 20) ... ok
+
+test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.51s
+```
+
+### Code navigation
+Rust provide a very powerful feature for us, that is code navigation in doc comments.
+
+Add following code to `src/lib.rs`:
+```rust
+// in lib.rs
+
+/// Add one to the given value and return a [`Option`] type
+pub fn add_three(x: i32) -> Option<i32> {
+    Some(x + 3)
+}
+```
+
+Besides jump into the standard library, you can also jump to another module in the package.
+
+```rust
+// in lib.rs
+
+mod a {
+    ///  Add four to the given value and return a [`Option`] type
+    /// [`crate::MySpecialFormatter`]
+    pub fn add_four(x: i32) -> Option<i32> {
+        Some(x + 4)
+    }
+}
+
+struct MySpecialFormatter;
+```
+
+### Doc attributes
+Below are a few examples of the most common `#[doc]` attributes used with `rustdoc`.
+
+### `inline`
+
+Used to inline docs, instead of linking out to separate page.
+
+```rust,ignore
+#[doc(inline)]
+pub use bar::Bar;
+
+/// bar docs
+mod bar {
+    /// the docs for Bar
+    pub struct Bar;
+}
+```
+
+### `no_inline`
+
+Used to prevent linking out to separate page or anywhere.
+
+```rust,ignore
+// Example from libcore/prelude
+#[doc(no_inline)]
+pub use crate::mem::drop;
+```
+
+### `hidden`
+
+Using this tells `rustdoc` not to include this in documentation:
+
+```rust,editable,ignore
+// Example from the futures-rs library
+#[doc(hidden)]
+pub use self::async_await::*;
+```
+
+For documentation, `rustdoc` is widely used by the community. It's what is used to generate the [std library docs](https://doc.rust-lang.org/std/).
+
+
+### Full Code
+The full code of package `doc-comments` is [here](https://github.com/sunface/rust-by-practice/tree/master/practices/doc-comments).