It was stated earlier that research method involves collection and analysis of data. Once the problem has been stated and the theories have been formed in the research methodology, it is necessary to collect the data, both in situ and remotely sensed, in order to progress toward a solution. If data is to be useful, it must be collected properly. Whatever the logic or research-type used, every problem will have different data requirements. This chapter is aimed to discuss the data and their collection/selection methods and related issues. First it will discuss the factors influencing the selection of remote sensing data for different types of applications; and then the ground truth and other ancillary data will also be addressed. However, the discussion in this chapter will not focus on instrumentations/sensors or scanning/imaging techniques to capture the remote sensing imagery. One may refer any standard remote sensing textbook for these topics.