The low-$Q^2$, low-$X$ region in electroproduction

The authors summarize existing experimental and theoretical knowledge of the structure function $F_2$ in the region of low $Q^2$ and low $x$. Constraints on the behavior of structure functions in the limit of $Q^2=0\mathrm{}$ are listed. Phenomenological low-$Q^2$ parametrizations of $F_2$ are collected, and their dynamical content is discussed. High-energy photoproduction and nuclear shadowing are described briefly. A recent update of the low-$Q^2$, low-$x$ experimental data and their implications is given.