We study the maximum absolute value of the determinant of matrices with entries in the set of $\ell$-th roots of unity; this is a generalization of $D$-optimal designs and Hadamard's maximal determinant problem, which involves $\pm 1$ matrices. For general values of $\ell$, we give sharpened determinantal upper bounds and constructions of matrices of large determinant. The maximal determinant problem in the cases $\ell = 3$, $\ell = 4$ is similar to the classical Hadamard maximal determinant problem for matrices with entries $\pm 1$, and many techniques can be generalized. For $\ell = 3$ we give an additional construction of matrices with large determinant, and calculate the value of the maximal determinant over $μ_3$ for all orders $n < 14$. Additionally, we survey the case $\ell = 4$ and exhibit an infinite family of maximal determinant matrices over the fourth roots of unity.