Informed by the theory of structures and materials science, design of civil structures is based on criteria that deal with the stresses and deflections in the structure. Consequently, assessment of stresses and deflections in real structures became one important aim of Structural Health Monitoring (SHM). In general, however, stress and deflection cannot be measured directly in real-life settings due to various limiting factors related to sensing technologies, site conditions, and properties of structural materials. Exceptions exist, and stresses and deflections can be directly monitored in very specific applications, but there are no technological solutions that can be generalized for that purpose. Strain is a parameter which is directly correlated to both stress and deflection. In addition, unusual structural behaviors (e.g., damage or deterioration) frequently have symptoms appearing as strain field anomalies. Hence, strain-based methods were among the first approaches applied in SHM. The first concepts of strain sensing were tackled in XIX century when Lord Kelvin demonstrated that electrical resistance of metallic conductors is related to their mechanical strain, and thus established the physical principle of a resistive strain gauge. However, the first strain sensor to be manufactured and applied in real-life settings is the vibrating wire sensor. Its creation was published in 1919 and the first applications were carried out in 1920's. Since then, two generations of strain sensors matured and became regular tools applied in strain-based SHM: discrete electrical sensors, and fiber-optic sensors (both discrete and distributed). While the former provided short-gauge sensors that established bases in local strain monitoring, the latter provided long-gauge and 1D-distributed sensors that greatly extended performance and applicability of strain-based SHM by enabling global structural monitoring and integrity monitoring. Current research aims to create a third generation of strain sensors that target expansion of integrity monitoring from 1D to 2D and 3D approaches, based on various technologies including 2D smart paints and surfaces, and 2D sensing skins and sheets, and 3D self-sensing materials, smart dust and sensing sand. This paper presents 100-year retrospective on strain sensing technologies employed in SHM, with aim to emphasize the milestones in creation of strain-based SHM and identify future directions.