Abstract: Many motivated extensions of the Standard Model of particle physics predict the existence of cosmic strings. Gravitational waves (GWs) originating from the dynamics of the resulting cosmic string network have the ability to probe many otherwise inaccessible properties of the early universe. In this study we show how the spectrum of GWsfrom a cosmic string network can be used to test the equation of state of the early universe prior to Big Bang Nucleosynthesis (BBN). We also demonstrate that current and planned GW detectors such as LIGO, LISA, DECIGO/BBO, and ET/CE have the potential to detect signals of a non-standard pre-BBN equation of state and evolution of the early universe (e.g., early non-standard matter domination or kination domination) or new degrees of freedom active in the early universe beyond the sensitivity of terrestrial collider experiments and cosmic microwave background measurements. While the focus of the talk will be on gauge cosmic strings, the potential GWs from global/axion strings will also be discussed which may provide a new probe for axion-like dark matter models.