36 is both the square of 6 and a triangular number, making it a square triangular number. It is the smallest square triangular number other than 1, and it is also the only triangular number other than 1 whose square root is also a triangular number.
It is the smallest number n with exactly 8 solutions to the equation φ(x)=n. Being the smallest number with exactly 9 divisors, 36 is a highly composite number. Adding up some subsets of its divisors (e.g., 6, 12 and 18) gives 36, hence 36 is a semiperfect number.
This number is the sum of a twin prime (17+19), the sum of the cubes of the first three positive integers, and also the product of the squares of the first three positive integers.