The entry and participation principles have been thoroughly reviewed by the IAAF and a new qualification system has been introduced.
The key element of the new system is that the IAAF shall establish the ideal number of athletes (and relay teams) to start in each event of the championships and shall ensure that such ideal numbers are met through a qualification system which, essentially, combines entry standards (only one standard) and invitations based on rankings.
As in the past, there are still some special qualification opportunities and it will still be possible for member federations without any qualified athletes to be represented with one unqualified athlete (subject to certain conditions).
Full entry and qualification system for Beijing 2015.