This chapter is devoted to the second algebraicK-group. We give two equivalent definitions, namely, in terms of the Steinberg group and in terms of the universal central extension of E(𝑅).We extend the long exact sequence associated to a pullback of rings and to a two-sided ideal beyond K1 to K2. The long exact sequence associated to a pullback of rings cannot be extended to the left to higher algebraic K-groups, whereas this will be done for the long exact sequence associated to a two-sided ideal later.