Band plans exist to guide radio amateurs on the optimum use of spectrum. For effective communications by radio, consideration is needed of many different types of transmission modes available today. Many of these do not comfortable share spectrum with other modes and so spectrum is used optimally if there is some segregation of modes.
IARU Member Societies contribute to the development and maintenance of bandplans. These can be accessed below.
After approval by the EC, the modifications of the Region 1 HF band plan become effective at 01 June 2016 as preliminary policy. Some modifications were recommended by the C4 Committee at the Interim Meeting in Vienna 2016 (UPDATE: Ratified at the General Conference in Landshut 2017)
The VHF band plans are kept under continuous review and the latest versions can be accessed via the above link