What makes a good solder? Is it flow or how it wets, hardness or ductility, strength or melt gap that is the most important quality, or is it a combination of all of these? There are many examples of jewelers who claim solder “A” is excellent but solder “B” is not so good, while others claim exactly the opposite. Why is this? Can we quantify the physical and mechanical properties of solders to predict which will be good and which will not? Can we predict from this information which solders are best suited to specific applications, or is it like the above statement and purely a subjective assessment with a “if it works for you, use it” philosophy? This paper will discuss the principles behind metal joining, measure the properties of solders and solder joints in various applications and determine if guidelines can be identified to predict which solders will work best in specific applications.