As the role of mechanical properties of cells and tissues is gaining high relevance in the study of a wide range of biological processes, Brillouin imaging has emerged as a promising tool for the characterization of biological samples in terms of their viscoelastic behavior. Traditional techniques for the characterization of biomaterials such as magnetic bead twisting, deformation microscopy, micro-rheology, or atomic force microscopy (AFM) either require contact, are destructive, or do not provide sufficient resolution. Conventional optical coherence elastography, which is a clinical tool that measures tissue biomechanics, is very sensitive to environmental vibrations 
. On the contrary, Brillouin imaging presents a contactless, label-free, non-destructive modality for probing biological samples in the GHz/micron scale and with great potential in clinical diagnosis.
The Brillouin light scattering (BS) effect was predicted by Léon Brillouin 
and Leonid I. Mandelstam 
independently in 1922 and 1926, respectively. However, there is controversy due to the possibility that L. I. Mandelstam had already presented this effect in 1918 
. L. I. Mandelstam joined Eugenii Gross to detect the BS experimentally, becoming the first person to observe the Brillouin effect and offer empirical confirmation 
The laser’s invention in the 1960s brought a light source powerful enough to reduce acquisition times and increase the resolution, resulting in hundreds of experimental works in the area of condensed matter and becoming a consolidated tool 
. Following the theoretical model presented by L. Brillouin and L. I. Mandelstam, Brillouin–Mandelstam scattering (for the sake of simplicity, it will be only denoted as Brillouin scattering (BS)) is based on the light’s interaction with collective fluctuations of density in the physical medium, provoking a change in the frequency of the scattered light; this effect is explained by a sort of Doppler effect in addition to the Bragg condition to obtain constructive interference.
The first examples of BS measurements on biological tissues were described in the late 1970s and early 1980s that demonstrated the power of this system as a research tool of high resolution and sensitivity 
. Following the development of the virtually imaged phased array (VIPA) spectrometers in 1996 and the introduction of Brillouin imaging in 2005, the topic has flourished into a prolific and fast-moving research field 
. During the last decade, a great variety of applications of BS to biology and biomedicine has been reported. BS has been tested as a screening and diagnostic tool and a monitoring resource for a wide range of biological samples at cellular and tissue resolution 
. This work aims to offer an overview of the development of BS-based techniques in biology, offering a comprehensive summary of the currently available literature and state-of-the-art instrumentation, and explores possible future outlooks (Figure 1
Figure 1. Timelines of the different applications presented in the review. (A) Square milestones represent publications related to human applications of Brillouin and have been classified into three groups: ophthalmology (EY), epithelial tissue (ET), and bone (BN). Circular milestones represent publications related to Brillouin microscopic studies regarding subcellular components (CC), biofilms (BF), and others (O). (B) Rhomboidal milestones represent Brillouin studies of animal tissues such as extracellular matrix (ECM), ephthalmological tissues (OP) such as the cornea or crystalline lenses, and others (O) that include neuronal, vascular, and oncological applications, among others. Triangular milestones represent publications related to Brillouin studies of animal organs such as bone (BN), spine (SP), embryos (EB), or others (O).