House of Lords authority in Tesco v Secretary of State for the Environment [1995] 1 WLR 759 has reinforced the well-established principle that judicial review will distinguish between relevant and irrelevant considerations pertaining to the exercise of a power, and leave the weighing of the relevant ones to the decision maker. It has also problematized the principle by insisting that relevant factors may adequately be taken into account even where the decision-maker allows them no influence (subject to challenge for irrationality). It is difficult to accept that a legislative identification of relevant factors need not influence the decision-maker, and the article begins by observing that this proposition would probably not have been accepted at the time of Roberts v Hopwood [1925] AC 578. It is argued in the article, however, that the proper influence of the relevant can be accommodated within the decision-making process while upholding the Tesco principle. The article therefore charts the activities which are logically implicit in a decision-making process including the consideration and weighing of relevant factors, and locates opportunities for legal challenge. The influence of the expressly relevant must be felt when the decision-maker has to decide whether other factors are impliedly relevant. It will equally be registered when a basis for the assessment of weight is established (weight being impossible to gauge without the prior formulation of criteria of influence). The process of weighing itself is shown to require two separate assessments; the first particular to each factor, the second comparative. This distinction allows explanation of those authorities which require substantial weight to be given to certain protected factors, and helps to define the nature of irrationality challenges to absurd weighing. In summary, the article takes a rigorous look at what weight is, how it is assessed, how it differs from relevance, and to what extent a flawed assessment of it can be challenged.