Rheology is a section of physics that studies, describes and evaluates the deformation and flow behavior of materials. Even materials with similar physical properties can have different types of behaviour: fluids flow at different speeds, and solids can be deformedto a certain extent. All substances, depending on their behavior, can be put in order from ideally viscous liquids to ideally elastic solids and in between highly viscous, semi-solid substances. This article briefly describes the fundamental principles and concepts of rheology, such as: viscosity (kinematic, dynamic), shear stress, shear rate, Newton's law of viscosity, yield point, shear deformation (shear strain), shear modulus, Hooke's law, complex shear modulus, storage modulus, loss modulus and damping factor (loss factor). The article refers to the application of all the above-mentioned physical concepts, tells about oscillatory and rotational tests and provides an information about the tools that can be used and describes the principles of their operation. One section deals with rotational tests: (controlled shear rate (CSR) and controlled shear stress (CSS)), flow curves and viscosity curves, and viscosity measurement. Also, the article describes in detail the study of viscoelastic behavior by oscillatory test and study of elastic behavior using two types of oscillatory tests: the amplitude sweeps and the frequency sweeps. At the end of the article you can find conclusions and our contacts information.