I will discuss how Gamma-ray bursts (GRBs), the most energetic explosions in the Universe, are ideal probes of extreme relativistic astrophysics. I will demonstrate that multi-wavelength followup observations spanning X-ray to radio waves are critical to detecting and characterizing these short-lived transients, and walk through specific examples of the riches revealed in our follow-up campaigns. I will show how we combine our observations with analytical modeling to measure the speed of GRB jets, and also to probe the acceleration of energetic electrons in relativistic shocks, the composition of the jets, and the mechanisms powering these energetic outflows. I will conclude with a discussion of the insights these and upcoming observations provide into the family of energetic transient phenomena that includes GRBs, and the role of global telescope networks in addressing unsolved questions and exploring new observational regimes in the field of high-energy time-domain astronomy.