Typically, when we think of a number system, we think of the base $10$ number system with digits $0, 1, \dots, 9$. For example, $1729 = 1\cdot 10^3 + 7 \cdot 10^2 + 2 \cdot 10^1 + 9 \cdot 10^0$ and $\pi = 3 \cdot 10^0 + 1 \cdot 10^{-1} + 4 \cdot 10^{-2} + 1 \cdot 10^{-3} \dots$. This is a special case of a very general construction. Here $d$ need not be an integer greater than $1$. It could be a real number, or negative, or complex, or a matrix. The set of digits could be any subset of real or complex numbers, vectors from $\mathbb{R}^n$, etc. In this talk we will survey several results of number systems, as well as connections to various areas as number theory, fractal geometry and quasi-crystals.