This chapter presents an overview of solder alloy systems that one is likely to encounter. It discusses the processes involved in the survey of solder alloy systems. The chapter describes the effect of metallic impurities on different solders. Extensive reference is made to phase diagrams to highlight particular points. An introduction to alloy constitution and phase diagrams designed for those with no background in the subject is presented, which covers the interpretation of phase diagrams and the associated terminology. The mechanism by which the intermetallics form and some of the implications of their presence in joints are discussed. The chapter describes the several key characteristics of eutectic alloys and factors most effective at depressing the melting point of solders by eutectic alloying.