To Sign in to your Yahoo! Mail account, click here.
Enter your Yahoo! ID and password in the spaces provided, and click the “Sign In” button.
NOTE: If you’ve signed in to your Yahoo! Mail account, or another Yahoo! service, within the past eight hours and you didn’t sign out or close your browser, your account remains accessible to you without signing in. Just click the Yahoo! Mail icon or click here.
You might see your Yahoo! ID in the Sign In screen if you’ve used the same computer to log in to Yahoo! Mail or another of Yahoo!’s personalized services (such as My Yahoo! or the Yahoo! Music Engine) on a previous occasion. To sign in to a different Yahoo! Mail account, just click the “Sign in as a different user” link, or sign out of the current account and sign in to the new one.
To prevent someone else from using the “Back” button or the History function, you should sign out completely everytime you sign in to your account.
To sign out completely, click the “Sign Out” link at the top of the page. This way, a person using the computer after you does not have access to your account. This is especially important if you use a public or shared computer.
[advt]International users: After signing out of your Yahoo! Mail account, click the “Sign Out Completely” link to completely sign out of all Yahoo! accounts. If you only sign out of Yahoo! Mail and don’t completely sign out of your Yahoo! account, a person using the computer after you can access many other parts of your Yahoo! account, including My Yahoo!, etc.
Yahoo! Messenger Note: When you sign in to Yahoo! Messenger, it might open a browser and automatically sign in to your Yahoo! account if you’ve selected “Automatically sign me in when I click a Yahoo! link in Yahoo! Messenger” in the Messenger – Preferences – General window. This selection opens your account and leaves it open unless you sign out, as described above. Make sure the box is not checked if you want to stop this from happening.[source]