The Balian--Low theorem (BLT) is a key result in time-frequency analysis, originally stated by Balian and, independently, by Low, as: If a Gabor system $\{e^{2\pi imbt g(t-na)\}_{m,n \in \Z}$ with $ab=1$ forms an orthonormal basis for $L2$ then $$(\int_{-\infty}^\infty |t g(t)|^2 dt) (\int_{-\infty}^\infty |\gamma \hat g(\gamma)|^2 d\gamma = +\infty$$ The BLT was later extended from orthonormal bases to exact frames. This paper presents a tutorial on Gabor systems, the BLT, and related topics, such as the Zak transform and Wilson bases. Because of the fact that $(g')^\wedge(\gamma) = 2 \pi i \gamma \hat g(\gamma)$, the role of differentiation in the proof of the BLT is examined carefully. We include the construction of a complete Gabor system of the form $\{see^{2\pi ib_mt} g(t-a_n)\}$ such that $\{(a_n,b_m)\}$ has density strictly less than 1, and an Amalgam BLT that provides distinct restrictions on Gabor systems $\{e^{2\pi imbt} g(t-na)\}$ that form exact frames.