Traditional single-input single-output (SISO) and multiple-input multiple-output (MIMO) information theory adopt spatially discrete modeling, which mismatches the continuous nature of the underlying electromagnetic (EM) fields. Therefore, it is essential to analyze the information-carrying capability of continuous EM fields, which motivates the research of EM information theory (EIT). In this talk, we investigate the basics and results of EIT. First, we review the fundamental analysis tools of classical information theory and EM theory. Then, we introduce the modeling and analysis methodologies of EIT, including continuous field modeling, degree of freedom (DoF), mutual information, and capacity analyses. After that, several EIT-inspired applications are discussed to illustrate how EIT guides the design of practical wireless systems. Finally, we point out several open problems of EIT, where further research efforts are required for EIT to construct a unified interdisciplinary theory.