This chapter is an introduction to the three main subjects that will be discussed in this book, i.e., structures, mechanisms, and machines. While the focus of this book is on mechanisms and machines, there are some interesting relations between them on the one hand and structures on the other hand that provide readers with a broad understanding of the subject. We will see later how the same methods that we use for the calculation of degrees of freedom of mechanisms and machines can be used for detecting if what we have in mind is a mechanism or a stationary structure with no ability to move. Same methods can help us detect if a structure is statically determinate or it has redundant members. Later we will also see how the static analysis methods that we use for structural analysis can provide first estimations for solution of dynamic problems. Such results can also be used for verifying the accuracy of dynamic solutions.