Publisher Summary This chapter focuses on the description and comparative evaluation of the different invasive techniques used for digesta collection in pigs. Most of the comparisons pertain to studies for determination of ileal amino acid digestibilities. In addition, the portal-arterial difference method is described and evaluated as an alternative to digesta collection methods for measuring nutrient uptake from the digestive tract. Because of the microbial metabolism of nitrogenous material in the large intestine, only a relatively small proportion of amino acid excretion in the faeces is directly related to the amino acids recovered at the distal ileum. Thus, depending on the amino acid and on the feedstuff, digestibility values obtained by the faecal analysis method overestimate or underestimate those obtained by the ileal analysis method. Most feed protein evaluation systems are based on ileal protein and amino acid digestibilities. In addition, net appearances of specific nutrients and metabolites in the portal blood can be determined by measuring the portal blood flow rate and the concentration difference between portal and arterial blood of the nutrient. Factors that modify gut-tissue metabolism, such as the enteric microflora and feed additives, also affect the composition of amino acids entering the portal circulation.