Subsets and power sets: A ⊆ B (A subset of B) if every element of A is in B. Proper subset A ⊂ B if A ⊆ B and A ≠ B. Number of subsets: If |A| = n, then A has 2^n subsets. Power set P(A) = all subsets of A. |P(A)| = 2^|A|. Example: A = {a, b}. Subsets: ∅, {a}, {b}, {a,b}. Total = 2² = 4. P(A) = {∅, {a}, {b}, {a,b}}. Solving: List subsets systematically (organize by size). For large sets, use formula 2^n. Exam tip: Null set and set itself always subsets. ∅ ⊆ A for all A. Practice: Finding specific subset count (proper subsets = 2^n - 1).