Magnetic reconnection is a plasma physical process in which a magnetic field is topologically rearranged, releasing free energy and imparting it to plasma matter. Though seemingly ubiquitous as a magnetic energy conversion channel in laboratory, space, and astrophysical settings, magnetic reconnection is by no means the same in these diverse environments. Astrophysical reconnection, in particular, is quite different from the type that occurs in the solar system. This difference stems from several unique and extreme properties of astrophysical plasmas. While plasmas on Earth and in nearby space are often non-relativistic, made of electrons and ions (e.g., protons), and non-radiative, astrophysical plasmas are quite frequently relativistic (in both the special and general senses), made of electron-positron pairs, and highly radiative – exhibiting non-trivial radiation-matter (e.g., QED) interactions.

In this dissertation, I focus on radiative relativistic reconnection regimes that prominently feature many of the above extreme effects. Here, the initial magnetic energy per particle exceeds the electron rest mass, enabling reconnection to accelerate relativistic particles. These particles go on to suffer strong radiative losses, emitting photons that may later be absorbed inside the system to produce pairs. Feedback from both direct radiative cooling and pair production contributes to a rich collective interplay between reconnection and high-energy radiative physics.

Throughout this work, I treat reconnection as an idealized physical process without explicit reference to many aspects of the global astrophysical environments where it unfolds. Instead, I obtain astrophysical relevance through the relativistic and radiative physics and through the parameter space that I explore. These mimic reconnection conditions in blazar jets – collimated relativistic outflows from active galactic nuclei traveling toward the observer. Adopting a simplified microphysical perspective enables me to model the plasma (including radiation) with high fidelity and from first principles, especially in the large portion of this work based on numerical simulations. In addition to revealing the nature and observable features of reconnection in blazar jets, the resulting insight sometimes actually constrains the global features of the jet itself, illustrating the utility of a bottom-up, physics-first approach.