In this chapter, you will explore the evolution of techniques for recording signals from the cortical structures of the brain using invasive electrodes. The discussion begins with an introductory overview of the brain and the nervous system, emphasizing the electrical nature of neuronal communication, which underpins various invasive brain signal recording methods. You will learn about the neurophysiological origins of key signal types, including spike or single-unit activity (SUA), multi-unit activity (MUA), local field potential (LFP), and electrocorticogram (ECoG). Additionally, this chapter covers the different types of electrodes used to capture these signals and concludes with a comparative analysis of non-invasive versus invasive brain signal recording techniques. Overall, this chapter lays the groundwork for understanding SUA, MUA, LFP, and ECoG, providing a base from which students can further explore this dynamic area of study.