Proteins are involved in many biological processes. Misfolded, truncated, or mutated proteins as well as over- or underexpressed proteins have been implicated in many diseases. Therefore, detection and quantification of proteins is extremely important. Conventional techniques such as the enzyme-linked immunosorbent assay, Western Blot, and mass spectrometry have enabled discovery and study of proteins in biological samples. However, many important proteins are present at low concentrations, rendering them undetectable using conventional techniques. Furthermore, limited ability to simultaneously measure multiple proteins in a sample has constrained our ability to fully study the proteome. In this review, we comprehensively discuss approaches for protein detection. We first discuss the fundamentals of proteins and protein assays, including affinity reagents, surface functionalization, assay formats, signal detection, and multiplexing. We then discuss the challenges with these methods and review existing methods for highly sensitive and multiplexed protein detection. Finally, we review recent advances in protein detection from the literature and discuss challenges and future directions.