This study examines various forms of permissive joinder, multiple claims and non-compulsory counterclaim mechanisms available under Consumer Law, especially from the perspective of multi-party strict liability or multiple-party claims against professionals using unfair commercial terms or unfair commercial practices. The study underlines the voluntary aspects characterising the joinder of consumers as plaintiffs, as well as the admissibility of the opting-in – opting-out dichotomy depends on the type of redress sought by the consumer as a pursuant litigating party. The litigant consumers may be represented in pursuing the avoidance of unfair contractual terms by legal entities, which are not mentioned by Law no. 193/2000, on unfair terms in consumer contracts between the qualified entities. In these hypotheses, the permissive joinder system is characterized as an opting-in system, based on the features of the common interest mandate agreement described in the Civil Code general provisions on the mandate contract provisions are dealing with the problematic of multiple-claim redress actions in which several consumers plaintiffs are represented by the attorneys, based on the same source of litigation against the professionals. Another reason for using the third-party intervention mechanism in consumer complaints is that of allowing a third party or a subsequent party to join a lawsuit engaged between the originating parties (consumer vs professional); where the claim emanates from the express assent of the intervenient, the procedural intervention will be voluntary, and it has been used in jurisprudence in litigious procedures involving consumers and credit professionals and also in actions in avoidance of unfair contractual provisions. Consumer associations who fulfil the legal representative requirements may introduce judicial claims supporting collective interests of consumers in the opting-out injunctive relief procedure, for instance in the field of unfair terms in consumer contracts; subsequently, after the judge had finally decided on the existence of unfair terms and ordered the professional to eliminate those unfair terms from the existing contracts, the pursuant consumers, who intend to compensate reciprocal payment obligations or to obtain redress for the past payments collected by the professional based on those unfair terms, may resort to individual actions in compensatory relief or to a voluntary joinder in the action introduced by other prejudiced consumers against the professionals. Another aim of this study is to trace the metrical features of the consumer’s right to procedural impleading; the consumer who justifies a legitimate interest may implead a third person, against the party could have introduced a separate claim on indemnity or (2) At its turn, the impleader may implead another person for the breach of warranty. Fundamentally, the impleader’s claim and the main claim will be discussed simultaneously. Nevertheless, should the discussions on the impleader’s claim unjustifiably delay the judgement on the main claim, the judge may decide on its disjunction in order to have the impleader’s claim judged separately.