How can I hide the Tool-bar in Emacs persistently?

Add the following to your init file (~/.emacs or _emacs or ~/.emacs.d/init.el):

(tool-bar-mode -1)

Emacs has a nice built-in customization interface.

Select Options › Customize Emacs › Specific Option, start typing tool, then hit TAB to see the options starting with tool. Choose tool-bar-mode then. Toggle its value to switch it off, and press Save for future sessions.

Just for future reference.

~/.emacs file with tool-bar, menu-bar and scroll-bar hidden

;; Disabling things
;;-----------------------------------------------------------------------
(menu-bar-mode -1) 
(toggle-scroll-bar -1) 
(tool-bar-mode -1) 

;;Note: If, after turning any of these off, you want to re-enable them for a single emacs window, you can do so by pressing Meta-x and then typing the command at the M-x prompt. (Copied from Web)
;;Example:
;;M-x tool-bar-mode
;;will turn the toolbar back on. 
;;-----------------------------------------------------------------------

Now, your emacs will look like this.

I agree with michael. But if you only add this line to your .emacs file, there will be errors when you run emacs in the command line mode. Thus, a better solution may be adding the following to you .emacs file:

(if window-system
    (tool-bar-mode -1)
)

so that, tool bar will be hidden only when you run it in GUI. Emacs in command line mode does not seem to have a tool bar.