Highly accurate and stable frequency standards are essential for telecommunications, and satellite navigation. Optical clocks that utilise fundamental atomic properties are set to become the future primary measurements standards for time and frequency replacing the caesium clocks in use today. Composite atomic clocks based on multi-species atoms have the potential to introduce even greater accuracy, stability, and reliability than existing single atom clocks. This project will investigate two-species composite atomic clocks based on established atomic clock species and explore new reference transitions in highly charged ions that promise to further improve the performance of clock systems.