Consider a large polymer molecule being dragged into a solvent made of relatively small molecules. The instantaneous movement of the polymer is difficult to predict due to collisions by a very large number of solvent molecules in random directions. As a result, the polymer undergoes random motion. The response of the polymer molecule to the random forces acting on it is additionally modulated by the presence of any externally imposed fields such as electric fields, thermal gradients, and gravitation. This phenomenon is ubiquitous in all situations dealing with polymers and colloidal particles dispersed in solutions. In this chapter, we shall summarize the key conclusions in general terms and implement these results in later chapters.