  One of the fascinating properties of the new families of two-dimensional
crystals is their high stretchability and the possibility to use external
strain to manipulate, in a controlled manner, their optical and electronic
properties. Strain engineering, understood as the field that study how the
physical properties of materials can be tuned by controlling the elastic strain
fields applied to it, has a perfect platform for its implementation in the
atomically thin semiconducting materials. The object of this review is to give
an overview of the recent progress to control the optical and electronics
properties of 2D crystals, by means of strain engineering. We will concentrate
on semiconducting layered materials, with especial emphasis in transition metal
dichalcogenides (MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$). The effect of strain
in other atomically thin materials like black phosphorus, silicene, etc., is
also considered. The benefits of strain engineering in 2D crystals for
applications in nanoelectronics and optoelectronics will be revised, and the
open problems in the field will be discussed.
