When to use session_start()?

[D4RK_HACK3R]

I've been going through some \[code\]$_SESSION\[/code\] examples and I'm a little confused, should i use \[code\]session_start()\[/code\] every time before I access or set \[code\]$_SESSION['variable']\[/code\] or should I only use \[code\]session_start()\[/code\] once per page or just once the user has successfully logged in?