I'm trying to install some packages on my server (Debian8) and for some of they, I always have dependencies problems.For example, I have executed this command:apt-get install jetty9And It answer that I've asked impossible things and say that the dependency libjetty9-extra-java is not satisfied . I can't understand why.Can someone tell me why I get such errors?Here is the error :Some packages could not be installed. This may mean that you have  requested an impossible situation or if you are using the unstable                                                                                             distribution that some required packages have not yet been created                                                                                             or been moved out of Incoming.                                                                                                                                 The following information may help to resolve the situation:                                                                                                   The following packages have unmet dependencies:                                                                                                                 jetty9 : Depends: libjetty9-extra-java (>= 9.2.14-1~bpo8+1) but it is not going to be installed                                                               E: Unable to correct problems, you have held broken packages.And this is my sources.list :#de://debian.mirrors.ovh.neb httpt/debian/ jessie main                                                                                                         #deb-src http://debian.mirrors.ovh.net/debian/ jessie main                                                                                                     deb http://security.debian.org/ jessie/updates main                                                                                                            deb-src http://security.debian.org/ jessie/updates main                                                                                                        # jessie-updates, previously known as 'volatile'                                                                                                               deb http://debian.mirrors.ovh.net/debian/ jessie-updates main                                                                                                  deb-src http://debian.mirrors.ovh.net/debian/ jessie-updates main                                                                                              # jessie-backports, previously on backports.debian.org                                                                                                         deb http://debian.mirrors.ovh.net/debian/ jessie-backports main                                                                                                deb-src http://debian.mirrors.ovh.net/debian/ jessie-backports main                                                                                            deb http://debian.mirrors.ovh.net/debian/ jessie main contrib non-free                                                                                         deb-src http://debian.mirrors.ovh.net/debian/ jessie main contrib non-free
Can't perform apt-get because of dependencies
debian;apt;software installation;dependencies;backports
Your jetty9 package is using the backports, as it can be seen by the bpo8 string.As you already have jessie-backports configured in sources.list, do:sudo apt-get updatesudo apt-get -t jessie-backports jetty9The -t jessie backports is a hint to apt for using the jessie-backports repository.Also check:https://packages.debian.org/jessie-backports/jetty9backports.debian.org Backports are packages taken from the next Debian release (called testing), adjusted and recompiled for usage on Debian stable. Because the package is also present in the next Debian release, you can easily upgrade your stable+backports system once the next Debian release comes out.