Book excerpt: The solar transition region, which spans the temperature range from about 20,000 to 1,000,000 K, separates the chromosphere from the corona. All the energy that heats the corona and powers the solar wind must pass through this part of the solar atmosphere. This book summarizes recent ultraviolet and extreme ultraviolet observations of the transition region, the empirical models derived from them, and the physical models that try to explain both the observations and the empirical models. The observational focus is on quiet solar transition region observations made with Skylab and subsequent rocket and satellite experiments. The book also presents a unified discussion of the analysis of ultraviolet and extreme ultraviolet spectroscopic data, including the determination of the emission measure and density and temperature diagnostics. This will be useful to astrophysicists who are confronting high-resolution ultraviolet and extreme ultraviolet data from astrophysical plasmas for the first time.