Cathode degradation often begins as subtle impedance growth before capacity fade is obvious. This tutorial shows how to diagnose degradation mechanisms using Electrochemical Impedance Spectroscopy (EIS), including dynamic EIS collected during battery cycling, and interpret overlapping processes with the Distribution of Relaxation Times (DRT), which converts frequency data into a distribution of time constants. We will also highlight emerging joint-domain (hybrid) impedance spectroscopy for solid-state batteries (SSBs), which combines time- and frequency-domain data to enable much faster measurements.