The aim of this special issue of Strain is to highlight current methods for the non- and minimally invasive evaluation of works of art. The trend of upgrading the inspection tools for artwork conservation has increased significantly in recent years, and most conservation laboratories as well as conservation organisations show an increased interest in new and effective tools that can solve everyday problems in the conservation field. In this issue, the focus on the non- or minimum invasivity of the methods is due to the ethical constraints set by the conservation scientists out of respect for the precious cultural heritage. It is commonly accepted that objects of fine arts such as paintings, sculptures, books, and historic buildings belong to the most important and valuable contribution of humanity tracing our identity, evolution, and progress. Therefore, their preservation deserves special attention across generations. An issue of main importance that a conservator has to consider prior to any restoration is the assessment of the structural and mechanical conditions of the artwork. To overcome conventional approaches such as the nail test and subjective visual monitoring, there have been great efforts in recent years to investigate and to implement efficient technologies to assist the restorers work and to objectify the assessment of the state of an artwork. An increasing need exists for non-invasive methods. Therefore, the significance of non-destructive and contactless inspection technologies dominates the discussions among experts in recent years. Many of the new instruments for both structural and analytical investigations are usually located in academic research institutes since they are usually invented there and require very specific expertise. Therefore, they are often far from being used by museums and cultural heritage researchers since the art pieces and monuments cannot easily moved to the location of the instruments. The implementation of portable instruments is therefore a further important challenge for all researchers and equipment manufacturers. Suitable experimental methods which were applied and tested explicitly in laboratory and in-situ are presented in this special issue. The eleven contributions from various countries and research institutions cover a broad range of inspection problems, technologies, instruments, and approaches. To them belong NMR techniques for depth profiling, Raman spectroscopy and chemometrics for investigating ageing effects on terpenoid resins, a coherent and low-coherence digital holographic interferometry set-up for structural diagnostics with example on a marquetry sample, digital shearography for the detection of sub-surface defects by applying a combination of FEM-simulation and measurement to support the process of fault classification, the investigation of Mediterranean wall paintings by its optical infrared emission, the development of a portable digital holographic speckle pattern interferometry system for remote-access monitoring in art conservation, photoacoustic imaging for uncovering the hidden content of layered documents, THz-technologies for time domain imaging of immovable cultural heritage materials, optical coherence microscopy for the evaluation of art objects made by marquetry technique, the use of digital image correlation for strain measurement in historic tapestries, and of the discrete Fourier transform to control strain in art objects by processing relative humidity data. Thus, this special issue is a snapshot of current activities in the field of minimally invasive artwork inspection and shows the wide bandwidth of modern technologies, approaches, and measurement objectives. The editors hope that these articles will contribute to the ongoing progress of optical inspection methods and will stimulate the cooperation between research groups and conservators to solve significant challenges in artwork protection and conservation. Stuttgart and Heraklion, February 2019 Wolfgang Osten and Vivi Tornari