The greatest common factor (GCF) of 18 and 36 is 18. One way to find the GCF of two numbers is to compare their prime factorizations. The prime factorization of 18 is 2 × 3 × 3, and the prime factorization of 36 is 2 × 2 × 3 × 3. The common prime factors are 2, 3, and 3. To find the GCF, we multiply these numbers together, which gives us 18.