Abstract: This paper is the first in a series of short tutorial articles loosely based on tutorial sessions given at the Royal Military College of Science to Masters of Science (MSc) students who are studying on defence technology courses. The purpose of the tutorials is to enable students to do first-pass (rough) calculations on various aspects of electro-optical system (electro-optical system analysis). This type of electro-optical systems analysis is typical of that which they may carry out in their potential future role as defence analysts. This first paper looks at calculating the power requirement of a laser and/or the sensitivity required of the detector for laser-beam riding, laser-warning receivers and laser-designation systems.