The Scout Motto of the Scout Movement is Be Prepared. In various languages, it has been used by millions of Scouts around the world since 1907. All the member organisations of the World Association of Girl Guides and Girl Scouts (WAGGGS) share this same motto.
Baden-Powell on "Be Prepared"
Of course, it is no coincidence that this motto can be shortened to B. P. and Robert Baden-Powell, the founder of the movement, used to shorten his surname into B.-P.
In the third part of Scouting for Boys Robert Baden-Powell explains the meaning of the phrase:
The Scout Motto is: BE PREPARED which means you are always in a state of readiness in mind and body to do your DUTY.
- Be Prepared in Mind by having disciplined yourself to be obedient to every order, and also by having thought out beforehand any accident or situation that might occur, so that you know the right thing to do at the right moment, and are willing to do it.
- Be Prepared in Body by making yourself strong and active and able to do the right thing at the right moment, and do it. 
"To do the right thing at the right moment" can be extreme:
- "Where a man has gone so far as to attempt suicide, a Scout should know what to do with him."
- "BE PREPARED to die for your country if need be, so that when the moment arrives you may charge home with confidence, not caring whether you are going to be killed or not" 
The motto of the Girl Guides is "Be Prepared". Why is this?
It is because, like the other Guides, you have to be prepared at any moment to face difficulties and even dangers by knowing what to do and how to do it. 
(The "other Guides" of this quote are the Khyber Guide Regiment.)
The following acrostic poem illustrates the Scout Motto:
In various languages
- Arabic: كن مستعدين (Kun Musta'idan)
- Armenian: Misht Badrast
- Azeri: Daima Hazir
- Belarusian: Napagatovye
- Chinese: 準備
- Croatian: Budi Pripraven
- Czech: Buď Připraven
- Danish: Vær forberedt
- Dutch: Weest Paraat / Wees bereid
- English: Be Prepared
- Esperanto: Estu preta
- Estonian: Ole Valmis
- Filipino (Tagalog): Laging Handâ (literally, always prepared)
- Finnish: Ole valmis
- French: Sois Prêt or Toujours Prêt
- Georgian: იყავი მზად (ikavi mzad)
- German: Allzeit bereit
- Greek: Έσω Έτοιμος
- Hebrew: היה נכון (heye nachon)
- Hungarian: Légy Résen
- Interlingua: Sempre Preste ("always ready")
- Irish: Bí Ullamh
- Italian: Sii Preparato (the latin expression Estote Parati is also used very often in Italy)
- Japanese: Sonae-yo Tsuneni
- Kyrgyz: Dayar Bol
- Latvian: Esi Modrs
- Latin: Estote Parati
- Lithuanian: Budek
- Macedonian: Budi Spreman
- Malay: Selalu Bersedia
- Maltese: Kun Lest
- Mongol: Belen Bol
- Norwegian: Alltid Beredt
- Pashto: Tayar Osay
- Persian: Aamaadeh Baash
- Polish: Czuwaj
- Portuguese: Sempre Alerta
- Russian: Будь готов (Bud' Gotov)
- Serbian: Budi Spreman
- Shqiptar: Ji Gati or Pergatitu
- Slovenian: Bodi pripravljen or Bud Pripavany
- Spanish: Siempre Listo or Siempre Alerta ("always ready", "always alert")
- Swedish: Var redo
- Thai: จงเตรียมพร้อม
- Turkmen: Daima Häzir
- Uighur: Tayyar Bol
- Ukrainian: Hotuis
- Vietnamese: Sắp Sẵn
- Baden-Powell, Robert. Scouting for Boys - Campfire Yarn No. 3 - Becoming a Scout.
- Baden-Powell, Robert. Scouting for Boys - Campfire Yarn No. 25 - Helping Others.
- Scouting for Boys, First Publication, #5, 12 February 1908, pp. 331-332. (not in later editions)
- Forbes, Cynthia. 1910... and then?.