For more detail, please check with the style book appropriate to your discipline or profession.
Use italics for emphasis, for unfamiliar foreign words and phrases, and for technical terms followed by definitions.
Examples: “He asked,” “She stated,” “According to Bronson,” or “As Shakespeare wrote.” Use a colon to introduce a quotation after an independent clause. While members of European cultures meet and shake hands as a gesture of greeting, members of Asian cultures bow to indicate respect.” He said, “I may forget your name, but I never remember a face.” History is stained with blood spilled in the name of “civilization.” Mullen, criticizing the apparent inaction, writes, “Donahue’s policy was to do nothing” (27).
Note: When parenthetical information falls within the closing punctuation, the information is shown to pertain only to that sentence.
The great march of “progress” has left millions impoverished and hungry.
For words used as words themselves or for technical or unfamiliar terms used for the first time (and defined), use italics. Use a comma to introduce a quotation after a standard dialogue tag, a brief introductory phrase, or a dependent clause. Nachas explains, “The gestures used for greeting others differ greatly from one culture to another.” D. Nachas explains cultural differences in greeting customs: “Touching is not a universal sign of greeting.
Instead, employ italics as they were originally meant to be used.
Quotation marks can be reserved for short works such as articles in periodicals, book chapters, short poems and songs, and other shorter items.
General Logic: Underscore was originally used on typewriters because it was impossible to use italics given that there was only one set of keys.
These days, underscore often means that a word is ‘hot’ on the computer, and with so much electronic transmission, I would advise against its use for other meanings than that.