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To promote the application of flexible strain sensors in the fields of athletics feedback, health monitoring, human–machine interface, and robotics, the design strategies of various strain sensors were reviewed according to their transition mechanisms from the input stimulus to the output signal. The transition mechanisms were categorized into four major types: resistance, capacitance, voltage (piezoelectric and triboelectric), and inductance/magnetism. The sensing performance of flexible strain sensors was summarized based on this categorization and compared according to the sensing mechanism. The study observed that the performance of resistance-type sensors depends on the sensor shape and structure of the conductive network. These sensors can detect various forms of strains and have a broad sensing range and high sensitivity. However, these require an external power source. Capacitance-type sensors display rapid response and high sensitivity to force variations. Thus, they can accurately detect subtle deformations with low energy consumption. However, these strain sensors also have certain disadvantages such as a marginal strain range and low repeatability. Voltage-type sensors are energy-saving devices that can directly convert mechanical energy into an electrical signal. However, they exhibit low sensitivity and cannot accurately detect subtle strains. Inductance/magnetism-type sensors can detect strains wirelessly. However, the signal-to-noise ratio is low, and the sensing range is narrow. Finally, the application and development prospects of flexible strain sensors were presented by describing their fabrication techniques and transition mechanisms.