I present studies of the X-ray emission from galaxy clusters in the context of understanding the multi-wavelength selection function and mass estimation of galaxy clusters. Clusters are gravitationally bound systems of dark matter, gas and galaxies, and are the largest gravitationally bound structures in the Universe. Constraining cosmology with cluster studies requires large, complete cluster samples with reliable masses over cosmologically significant survey volumes. Such samples are only just becoming available with new surveying telescopes that have optimal sensitivity for cluster finding, and some with the additionally useful property of finding clusters largely by their mass, which is the cosmologically significant cluster property. These surveys have increased the need for obtaining large, well-understood samples of clusters for characterizing their selection. Ongoing work in this field aims to determine the selection and limitations of the four ways of observing clusters and measuring their masses (optical/infrared, X-ray, weak lensing, or Sunyaev-Zel'dovich effect) using both large and small samples. In this context, I have studied small cluster samples in the X-ray and compared their properties to those determined through three different cluster selection methods: through weak lensing selection, optical selection, and selection through the Sunyaev-Zel'dovich effect. In this thesis I begin in the introduction with a brief overview of the current cosmological picture and how clusters help to constrain cosmology. In the subsequent chapters I describe my work in the X-ray, aimed at better understanding clusters selected through the different methods. I conclude with a summary of how my follow-up and other multi-wavelength studies have illuminated cluster selection and also with comments on the persisting need for similar studies in the near future.