Technological thin film is widely used in modern microelectronics, optoelectronics, photovoltaic power, catalysis, and sensitive devices. Thin films take on very different physical characteristics from bulk materials because of their nano-scale thickness. These unique properties are described in this chapter, which is an introduction to characterization methods (including structure, composition, and electronic behavior) capable of estimating the number of atoms and atomic spatial distribution at microscale, mesoscale, and nanoscale levels. This section aims to offer readers an understanding of the different techniques used for characterizing thin films. It covers structure (XRD, TEM, AFM), composition (XPS, SIMS, RBS), and optical/electronic (UV–Vis-NIR spectroscopy, ellipsometry, Hall effect measurements). The discussion includes synchrotron- and time-resolved methods, such as X-ray absorption spectroscopy (XAS), scanning transmission X-ray microscopy (STXM), and ultrafast spectroscopy, which provide element-specific, nanoscale, and femtosecond-resolved local structure and carrier dynamics. In-situ and operando characterization are emphasized in observing in real-time the growth processes, phase transformation processes, and degradation mechanisms in operational conditions. In addition, correlative and multimodal techniques are emphasized with the aid of artificial intelligence (AI) and machine learning in acting as game-changing methodologies for handling comprehensive datasets and revealing latent structure–property relations.