Illustration Style Guide is documentation defining a brand's custom illustration visual language. It includes perspective rules, character design standards, stroke techniques, color palettes, composition patterns, and usage examples. The guide enables teams (internal illustrators, freelancers, agencies) to produce consistent illustrated content aligned with brand identity. Practitioners research brand personality, create sample illustrations exploring techniques, document rules, and refine iteratively based on real-world usage. The guide evolves as the brand grows (new character types, new composition patterns).