We can trace star formation through a broad variety of observations: photospheric emission from massive stars in the ultraviolet, dust emission in the infrared from grains heated and excited by energetic photons, hydrogen and metal recombination lines from the optical to the infrared, and even free-free continuum and synchrotron emission in the radio domain. The first and foremost constraint for astronomers in estimating SFRs is the ability to obtain adequate observations. For instance, distant galaxies may have emission lines shifted beyond the near-infrared, making them inaccessible from the ground, or the object may be too faint for its free-free emission to be detected. The nature of the observed galaxy and the available instruments therefore strongly guide how we can measure star formation. In the context of this chapter we concentrate on detailing how we can use any observation in star formation tracing bands to measure the SFR as reliably as possible. We will start with theoretical considerations to understand the impact if the assumptions behind each SFR estimator and then discuss the observational constraints.

Dust impacts observations of stars and gas in galaxies by absorbing and scattering photons. Correctly accounting for the effects of dust allows for more accurate studies of a galaxy's stars and gas while also enabling the study of the dust grains themselves.The impact of dust on measurements of individual stars in a galaxy can be straightforwardly modeled as extinguishing the stellar light. Dust extinction towards a star is defined as the combined effect of absorption of photons and scattering of photons out of the line-of-sight towards the star. For integrated measurements of regions of galaxies or whole galaxies that contain multiple stars intermixed with dust, the effects of dust are termed attenuation and are harder to model. Integrated measurements include stars extinguished with different amounts of dust and scattering of photons into the measurement aperture. The infrared dust emission powered by the absorbed photons provides a vital measurement of the amount of energy absorbed by dust. This infrared measurement is not possible for individual stars butprovides an important constraint in modeling the effects of dust on integrated measurements. The aim of this chapter is to provide the details of dust extinction, attenuation, and emission and recommendations for how to model the effects ofdust on observations.

Active Galactic Nuclei (AGN) are thought to play major role in the evolution of galaxies, impacting both star formation and gas accretion onto galaxies.Clearly there is a need to determine the star-formation rates(SFRs) in AGN host galaxies in order to understand and disentangle the growth of SMBH and their hosts. At the same time, contribution of their non-stellar emission in the measured emission from a galaxy impacts all SFR tracers. In this chapter we present the different types of AGN and the main mechanisms responsible for their emission in different wavebands. We discuss the methods used to identify whether a galaxy hosts an AGN and the available techniques to determine the fractional contribution of the AGN to various SFR tracers.

In many ways the study of resolved stellar populations is the bestmethod for exploring properties of stellar populations. However, the method requires measurements to be obtained for individual stars, and this rapidly becomes challenging as the distance to extragalactic systems increases. The depths of resolved stellar samples in galaxies are primarily limited by the levels of stellarfluxes and effects of crowding. Currently most resolved stellar population studies are constrained to galaxies within a distance of about 20 Mpc. Fortunately, the short-lived massive stars, whose numbers trace SFRs, are luminous and thus among the most readily observed, especially when they are not obscured by interstellar dust. The number of stars above a fiducial luminosity in a set of spectroscopic band-passesare counted and corrected for incomplete sampling. The distribution of these stars is then compared to expectations of stellar population models to derive estimates for the observed mass in the form of stars detected in the data. Further modeling provides an interpretation in terms of stellar masses within age bins. In this chapter we provide a brief overview of the history and some of the techniques used to derive star-formation rates (SFRs) and the associated star-formation histories of galaxies through observations