Apart from offering a systematic and insightful framework for modeling and analysis of multi-physics systems, port-Hamiltonian systems theory provides a natural starting point for control. Especially in the nonlinear case it is widely recognized that physical properties of the system - such as balance and conservation laws and energy considerations - should be exploited and respected in the design of control laws which are robust and physically interpretable. Port-Hamiltonian Systems Theory provides a concise and easily accessible description of the foundations underpinning the subject, and goes on to emphasize novel developments in the field that will be of interest to a broad range of researchers. The tutorial style makes it suitable for use in a course and by students.