Tackifier resins (TRs) are of considerable significance in numerous adhesive applications, where they are compounded with elastomers as part of the adhesive formulation. TRs serve as a solid solvent for the elastomers. The low molecular weight fractions of the elastomer are dissolved by the tackifier, while the high molecular weight fractions exist as partially solvated aggregates. This system behaves as reinforcing agent embedded in a tacky semi-solid matrix (1) and has a lower modulus and higher glass transition temperature than the unmodified elastomer. The lower modulus promotes bond formation and, coupled with the higher T-g, allows the system to resist bond rupture. Polymers typically tackified include ethylene vinyl acetate, styrene-butadiene rubber, natural rubber, polyisobutylene, styrene block copolymers and acrylics. [GRAPHICS] There are two main classes of TRs: synthetic materials and natural derivatives. The class of synthetic materials can further be divided into the sub-classes of petroleum resins, pure monomer resins and phenolics. Natural derivatives are based on either polyterpenes or rosins. The manner in which TRs are incorporated into the formulations is dependent on the form of resin used, the nature of the adhesive product and the properties desired. Solvent-based, hot melt and waterborne adhesives may utilize resins of all types and forms. Resins may vary from oils to solids melting at temperatures > 100 degrees C. For waterborne adhesives, TRs may be added directly if they are oils or as emulsions for higher melt point tackifiers. Methods of emulsification will be reviewed in this paper, with major emphasis on hydrophobically modified polyacrylic acid (HMPAA or Pemulen* resin) stabilized emulsions. Advantages of using HMPAA over other emulsifiers are examined. A case study of the emulsification of a petroleum hydrocarbon resin is presented.