  The Cherenkov Telescope Array, CTA, will be the major global observatory for
very high energy gamma-ray astronomy over the next decade and beyond. The
scientific potential of CTA is extremely broad: from understanding the role of
relativistic cosmic particles to the search for dark matter. CTA is an explorer
of the extreme universe, probing environments from the immediate neighbourhood
of black holes to cosmic voids on the largest scales. Covering a huge range in
photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of
performance with respect to current instruments.
  The observatory will operate arrays on sites in both hemispheres to provide
full sky coverage and will hence maximize the potential for the rarest
phenomena such as very nearby supernovae, gamma-ray bursts or gravitational
wave transients. With 99 telescopes on the southern site and 19 telescopes on
the northern site, flexible operation will be possible, with sub-arrays
available for specific tasks. CTA will have important synergies with many of
the new generation of major astronomical and astroparticle observatories.
Multi-wavelength and multi-messenger approaches combining CTA data with those
from other instruments will lead to a deeper understanding of the broad-band
non-thermal properties of target sources.
  The CTA Observatory will be operated as an open, proposal-driven observatory,
with all data available on a public archive after a pre-defined proprietary
period. Scientists from institutions worldwide have combined together to form
the CTA Consortium. This Consortium has prepared a proposal for a Core
Programme of highly motivated observations. The programme, encompassing
approximately 40% of the available observing time over the first ten years of
CTA operation, is made up of individual Key Science Projects (KSPs), which are
presented in this document.
