The purpose of this chapter is to provide the reader with some tools for studying and understanding the sensory systems in general. The notions of absolute threshold and difference threshold are defined, and a series of classic methods for estimating these thresholds are described: methods of limits, of adjustment, and of constant stimuli. For each method, the absolute and difference thresholds correspond to an operational definition. The signal detection theory is also described, which allows to make a fundamental distinction between sensitivity to sensations and the way of setting a response criterion. The chapter also offers the description of laws in the field of psychophysics. One is the discrimination law of Weber, which relates two physical quantities: the difference threshold and the magnitude of the sensory continuum under investigation. Another law, referred to as the psychophysical law or Stevens’ power law, describes the relationship, for a given dimension in a sensory modality, between the psychological magnitude and the physical magnitude.