Beyond the Standard Model with Pulsar Timing Gravitational wave astronomy represents a unique opportunity to probe fundamental physics at energy scales beyond the reach of current and future colliders. The recent analysis by the NANOGrav collaboration based on their 12.5-year pulsar-timing data set has shown evidence for a common red process which is compatible with a stochastic background of gravitational waves. In this talk, we will discuss the interpretation of this signal in terms of physics beyond the Standard Model with focus on cosmic strings, highly-energetic topological defects arising as a consequence of cosmological phase transitions in the early Universe.