The central objective of this chapter is to establish the integral representation theorem of displacement, as the basis of the source representation theorem introduced in Chap. 3. The displacement representation theorem can be seen as a direct consequence of Betti’s reciprocity theorem, provided one set of forces takes a specific form that leads to the emergence of Green’s functions. Betti’s reciprocity theorem itself is derived from the equations of elastic motion–also known as the equations of elastodynamics–using two different sets of forces. Therefore, the equations of elastodynamics form the foundation. To introduce these equations, it is necessary to discuss concepts like stress and strain, as well as related field theory knowledge. This reversal of order is the approach taken in this chapter.