In many cases, the analytical understanding of the complex nonlinear behaviour of plasmas is simplified by the fact that the plasma dynamics is low-dimensional. Then, quite universal descriptions, which can predict quite sophisticated phenomena, are available. This aspect, and the corresponding appearance of spatially coherent structures, are first discussed for the single-mode dynamics in a weakly unstable (or marginal) regime. The procedure is then generalized to a few modes. The following examples are used to point out the benefits from a low-dimensional description: ionization wave instability, Pierce instability, interchange instability and electron drift instability. Special emphasis is put on the following aspects. Low-dimensional dynamics is quite rich and allows one to apply modern control mechanisms. Coherent structures are present in the dynamics and may contribute to the understanding of transport scalings. The overview is concluded by pointing out some similarities to high-dimensional situations. In the latter, structures (singularities) also have a signature in the dynamical behaviour. They allow us to understand more deeply certain scaling behaviour.