Immunoglobulins are a heterogeneous group of proteins. It naturally follows that the strategies for purifying them are diverse and numerous. A good knowledge of their respective physiochemical properties will obviously make the task easier. The choice between using polyclonal and/or monoclonal antibodies will govern the basic approach. Each approach will present its own advantages/disadvantages including cost, ability to produce a high yield, quality, and a need for standardization. The context in which the antibodies will be used is another important aspect to consider. When the demand is for establishing "ultrasensitive" assays, optimal purity and specificity is obviously required. This chapter will focus on the purification of mammalian IgG from polyclonal (i.e., rabbit) and monoclonal (i.e., mouse sources). IgG is the principal immunoglobulin constituent of mammalian sera. In older animals, it may well represent >80% of the total Ig concentration, because of its higher rate of synthesis and longer half-life.