Piccoli e grandi numeri in fisica

Numbers are often viewed as a matter for end-of-chapter problems. Actually numerical estimates of physical quantities are of utmost importance: significant examples of them should always accompany the discussion of physical laws. When is the measure of a physical quantity merely conventional, and when not? Could fundamental constants be entirely disposed with? Examples are given of how relevant properties of a system can be inferred from pure numbers. Pure numbers formed with fundamental constants lead into matters still under current research.