There are two methods of esterification that are known and are group specific—namely, reaction with methanol-HCl or ethanol-HCl and reaction with derivatives of diazoacetic acid near pH 5. This chapter focuses mainly on new methods that have developed for reagents that are relatively specific for carboxyl groups. The other category of reagent, the reactive halogen compounds, is given a more cursory treatment because the particular structure of the binding site in the enzyme must be of primary importance in each case. Esterification may be regarded as a special case of alkylation. It is therefore likely that particular compounds, which would ordinarily be classified among typical alkylating reagents, are found to esterify uniquely situated carboxyl groups in enzymes and other proteins as the result of specific interactions. The chapter describes methods for determination of the extent of esterification. The investigation of the esterification of lysozyme by triethyloxonium fluoroborate showed, indeed, that there are two carboxyl groups that have an enhanced reactivity toward this reagent. There are growing number of enzymes are known that are inhibited in each case by esterification with an active halogen compound. Esterification of carboxyl groups in enzymes is observed with very different types of reagents, the derivatives all have the ester linkage in common.