  The expansion of variable generation has driven a transition toward a 100\%
non-fossil power system. New system needs are challenging system stability and
suggesting the need for a redesign of the ancillary service (AS) markets. This
paper presents a comprehensive and broad review for industrial practitioners
and academic researchers regarding the challenges and potential solutions to
accommodate high shares of variable renewable energy (VRE) generation levels.
We detail the main drivers enabling the energy transition and facilitating the
provision of ASs. A systematic review of the United States and European AS
markets is conducted. We clearly organize the main ASs in a standard taxonomy,
identifying current practices and initiatives to support the increasing VRE
share. Furthermore, we envision the future of modern AS markets, proposing
potential solutions for some remaining fundamental technical and market design
challenges.
