The Herschel Space Observatory (aka Herschel) was successfully launched on 14 May 2009. It has been designed to provide the astronomical community with a facility to carry out routine astronomical observations for a period of at least three years. General information about the Herschel mission, the observatory, its instruments, and the ground segment is provided in the Herschel Observers' Manual.

The majority of Herschel's observing time is made available to the astronomical community by the traditional route of calls for observing proposals, followed by peer review, in response to Announcements of Opportunity (AO), issued by the Herschel Science Centre (HSC). The observing time consists of Guaranteed Time (GT) and Open Time (OT). The GT (32% of the available time) is owned by contributors to the Herschel mission, mainly the science payload consortia. The remainder time is OT, that is awarded in a standard competitive manner to investigators worldwide. All observing proposals are reviewed by the Herschel Observing Time Allocation Committee (HOTAC).

This is the first in-flight Herschel AO, and currently open time (OT1) observing proposals are solicited for. The guaranteed time part (GT1) has already been conducted. There is one year's worth of observing time available to be allocated in OT1: 6592 hours.

There are two kinds of proposals: 'large' proposals for programmes requiring observing time in excess of 100 hours, and 'normal' proposals, for observing programmes requiring up to 100 hours. All proposals will be subjected to scientific and technical evaluation of the proposed observations, and checking with respect to the 'Reserved Observations List' (ROL) which together with the duplications policy is considered part of the AO package.

There is the possibility - under certain conditions - to apply also for XMM-Newton observing time as described in Section 4.5, “OT1 joint Herschel/XMM-Newton observing time”.

Responding to the AO is a two-phase process. Responses to this call consist of so-called Phase 1 proposals. After peer review by the HOTAC, successful proposers will have the opportunity to update their observations under the technical advise of the HSC according to the recommendations made by the HOTAC entering them into the Herschel database in their final version (Phase 2).

Another call will be issued in about a year's time as described in Chapter 10, Further calls for proposals and will be accompanied with its corresponding Policies and Procedures document.