In rheology, authors probe the response of a material as a strain deformation or stress is applied. This chapter discusses shear deformations, and the concepts and methods, which are directly applicable to extensional flows and deformations. The response of a material to deformations, both the characteristic timescales of the material and the timescale of the deformation are of key-importance. The rheological-behavior becomes particularly rich if the timescale of deformation and the typical relaxation time are comparable. The simplest rheological-experiment for testing the time-dependent material response is the stress relaxation test, which also exemplifies the basics of elastic, viscous, and viscoelastic response. In steady shear-rheology, the viscosity of a material as a function of shear-rate is measured. In Fourier transform rheology, the anharmonic stress response in large amplitude oscillatory shear (LAOS) is analyzed in terms of Fourier series. The chapter also discusses the commonly used rheometers and their advantages and limitations.