In this survey article we discuss the problem of determining the number of representations of an integer as sums of triangular numbers. This study yields several interesting results. If n > 0 is a non-negative integer, then the nth triangular number is 7", = n(n + 1)/2. Let k be a positive integer. We denote by 6k(n ) the number of representations of n as a sum of k triangular numbers. Here we use the theory of modular forms to calculate 6 k (n). The case where k = 24 is particularly interesting. It turns out that, if n > 3 is odd, then the number of points on the 24 dimensional Leech lattice of norm 2n is 212(212 -1)624(n -3). Furthermore the formula for (~24(r/) involves the Ramanujan z(n)-function. As a consequence, we get elementary congruences for z(n). In a similar vein, when p is a prime, we demonstrate 624(P k -3) as a Dirichlet convolution of alt (n) and r(n). It is also of interest to know that this study produces formulas for the number of lattice points inside k-dimensional spheres.Consequently, the values of rk(n) are the formula coefficients of the power series Ok(q) = ~ rk(n)q". n>~O