Single sign-on (SSO) helps users to cope with many online services that require authentication. Systems such as OpenID and SAML-based Shibboleth offer federated identity management where an Identity Provider authenticates the user on behalf of the services. Much research concentrates on making authentication stronger, preventing phishing and making the systems more user friendly but less attention has been paid to the termination of the authentication sessions i.e. logout. It is, however, equally important that the sessions do not remain open when, for example, a student using shared computers in a university library leaves the workstation. In this article, we describe challenges related to logout in federated identity management on web based services and give guidelines for implementing reliable logout from services that use single sign-on.