Acoustics are frequently used as a non-invasive imaging technique, mainly because the employed wave field is harmless and the required instrumentation is relatively cheap as compared to other imaging modalities. Reconstructions from the object of interest are obtained using a wide variety of imaging methods. The simplest one is in literature referred to as Synthetic Aperture Focussing Technique (SAFT). With this method, the boundaries of the objects / artefacts are reconstructed by constructive and destructive interference of the recorded wave-fields that are back-projected into the domain of interest. Although this method yields sharp images for wide-band signals it comes with limitations. SAFT only construct reflectivity images that do not reveal any quantitative information about the actual medium properties. In addition, the resulting images tend to degrade when the data exhibits many multiples or (large) phase shifts caused by significant speed of sound variations in the media. To overcome aforementioned limitations, a variety of reconstruction methods have been introduced with increasing complexity. Among the most complex ones are full-waveform inversion. These methods use the acoustic wave equation to reconstruct the unknown medium properties from the wave field measured at the boundary of the domain of interest. During our presentation, we will explain the fundamentals of full-waveform inversion. In addition, we show several techniques that can be used to regularize the inverse problem. These methods have been developed in close collaboration between the universities in Netherlands and Colombia.