The greatest common factor (GCF) of 12 and 18 is 6. There are different methods to find the GCF of two numbers, including prime factorization, long division, and Euclidean algorithm. One way to find the GCF of 12 and 18 is to compare the prime factorization of each number. The factors of 12 are 1, 2, 3, 4, 6, and 12, while the factors of 18 are 1, 2, 3, 6, 9, and 18. The greatest common factor is the largest number that appears in both lists, which is 6. Another method is to use long division, where 18 is divided by 12 repeatedly until the remainder is 0. The divisor at this point is the GCF, which is 6.