Jimmy Kimmel tweeted that "Robin was as sweet a man as he was funny. If you're sad, please tell someone."
And Lady Gaga said, "Rest in Peace + Laughter Robin Williams. Perhaps this tragedy will shed light on the seriousness of depression."
"He will help so many young people understand that there are many, many scores people who suffer from depression and addiction," said Dr. Harris Stratyner, an Addiction Psychologist.
A spokesperson for the comedian confirmed he had been battling severe depression. And for years he struggled with drug addiction.
"Alcohol is especially dangerous for people like myself, alcoholics or as you can say ethanol challenged, whatever you want to call it," Williams had said.
The legendary actor was back in rehab earlier this summer, confirming the demons he could not escape.
"Not caused by anything, it's just there, lays in wait for the time when you think, it's fine now, I'm okay, and then, and then, the next thing you know, it's not okay," Williams said.
But addiction psychologist Dr. Harris Stratyner says treatment works.
"There are thousands and hundreds of thousands of people with depressions and with addiction who go for treatment and they live happy, healthy, productive lives," Dr. Stratyner said.
Depression defies stereotypes affecting anyone, including comedians.
"One of the defense mechanisms that very intelligent people use is humor. So it makes sense you would defend against sadness with humor," Dr. Stratyner said.
With more than 200,000 shares, a Facebook post of a well-known quote by Maria Shriver reminds us to "Be kind. "For everyone you meet is fighting a battle you know nothing about."