In this chapter, we present the basics of AMReX. The implementation source codes are in amrex/Src/Base/. Note that AMReX classes and functions are in namespace amrex. For clarity, we usually drop amrex:: in the example codes here. It is also assumed that headers have been properly included. We recommend you study the tutorial in amrex-tutorials/ExampleCodes/Basic/HeatEquation_EX1_C while reading this chapter. After reading this chapter, one should be able to develop single-level parallel codes using AMReX. It should also be noted that this is not a comprehensive reference manual.