In this paper, five sensing mechanisms including capacitive, piezoresistive, inductive, piezoelectric, and optical are reviewed in details with representative literature. Basic functions of normal force, shear force, shear angle, torsional angle, rotational angle; and characteristics in sensitivity, dynamic window, and spatial resolution are discussed and categorized clearly in this article. In addition, materials used in the flexible force sensors for electrodes, substrates, and deformable dielectrics are highlighted and summarized. Furthermore, structural design, specifications of sensitivity, spatial resolution, dynamic range and cyclic life are also unambiguously outlined. Meanwhile, key design considerations are listed and evaluated in this review to indicate major contributions to the characteristics of the flexible force sensors. Notable challenges, potential business, and examples commercial players in the market are listed at the end of this manuscript. This article offers a stepping stone for those who plan to enter the flexible force sensor fields by revisiting the outcomes from the past decades.