How many numbers greater than 1000 can be formed using the digits 0, 1, 2 and 3 (repetition of digits is not allowed)?

To form a number greater than 1000 using digits 0, 1, 2, 3 without repetition, we need 4-digit numbers (since using all 4 digits is the only way to get a number greater than 1000 from these digits). The thousands place cannot be 0, so it has 3 choices (1, 2, or 3). The remaining 3 positions can be filled with the remaining 3 digits in 3! = 6 ways. Therefore, total = 3 × 6 = 18 numbers.