This thesis describes cysteine-selective methods for protein labeling and peptide stapling, with emphasis on arylation via nucleophilic aromatic substitution (SNAr). Proteins are Nature's most versatile biomolecules, made from a toolbox of 20 canonical amino acids, and fulfil a multitude of functions. Chapter 1 gives an overview of existing methods for chemical labeling and modification of the individual amino acid types of proteins, and the trends and challenges within this area. Chapter 2 describes how the SNAr reaction was used in this PhD project to install trans-cyclooctene (TCO) moieties on human serum albumin, a protein of interest as a potential tumor-targeting vector, for potential pre-targeting theranostics using tetrazine (Tz) ligation. Chapter 3 describes how SNAr bis-arylation was used to staple (cyclize) Cys-containing peptides with fluoroaryl compounds in ortho-, meta-, and para-geometries, and how this method could be used to tune, depending on the substitution geometry, the structure of the peptides. Chapter 4 describes the efforts to expand the peptide stapling method to proteins and at the same time use it as a way of introducing labels or cytotoxic payloads. Chapter 5 describes how silver coordination to cysteines was used as an alternative to SNAr bis-arylation to cyclize peptides and tune their secondary structures, in this case by inducing α-helicity. Chapter 6 describes a method of introducing γ-, δ-, and ε-lactams at the C-termini of peptides synthesized on solid phase support.