* In print: Use *italics*. This is the standard style guide recommendation (e.g., Chicago Manual of Style).
* In typed text (e.g., email, essay, document): Use *italics*. Again, this is the standard. If italics aren't available or easily implemented, you can use quotation marks, but italics are preferred.
* Handwritten: Underlining is acceptable in handwritten text as a substitute for italics.
Example:
Incorrect: "I just finished reading the book 'The Great Gatsby'."
Correct: "I just finished reading the book *The Great Gatsby*." (or, if handwritten, "I just finished reading the book The Great Gatsby.")