To explore the star formation history of galaxies, we should extract physical information by comparison between the theoretical model and observed quantities. With the theoretical tools prepared in Chapters 6 and 7, we can estimate the star formation properties from observations. In this chapter, we start with \({\text{ H }\alpha }\) , the most basic SFR estimator. We also introduce the nonionizing ultraviolet (UV) continuum as another direct SFR indicator. Performance of these primary indicators is limited by the ubiquitous existence of dust in galaxies. We discuss the attempt to “correct” the dust extinction and see how difficult it is. Then, dust emission, another path to estimate SFR, is discussed. Combining the UV-based and dust-based estimators gives a category of hybrid estimators of the SFR. They become the mainstream of the SFR estimation because of their stability and robustness. Though the UV–dust hybrid estimator works excellently, still the dust continuum flux density is harder to observe with current IR/submm instruments. A convenient method is proposed to overcome the lack of IR data. This makes use of the IR excess–UV slope relation, or in short IRX– \(\beta \) relation, to estimate the dust extinction \(A_{\lambda }\) from the UV spectral slope. We discuss the underlying physics, efficiency, and limitations. Closely connected to the IRX– \(\beta \) relation is the estimation of the attenuation \(A_\lambda \) . We introduce the method to estimate \(A_\lambda \) from the IRX. Based on these primary estimators, we can calibrate secondary SFR estimators. We introduce optical forbidden lines, IR hyperfine structure lines, X-ray emission, and radio continuum. We also discuss the usefulness of the SED fitting as an SFR estimator. The specific SFR (SSFR) and related observable quantities are also introduced.