Interference by public authority with individual rights, even when grounded in law, does not dispense with the need to justify the limits of such interference. As a fundamental principle, proportionality constrains public authority in determining those limits. Evidence gathering is one sphere in which such authority is exercised, since crime detection involves measures that intrude upon rights. Using a descriptive-analytical approach, this study asks what components underpin the principle of proportionality and what place this principle occupies in determining the justified limits of interference with individual rights for evidence gathering in Iranian criminal procedure. The findings indicate that proportionality makes the legitimacy of public interference contingent upon the legitimacy of the aim, the suitability of the means, necessity, and a balance between the importance of the aim and the intensity of interference with the right concerned; its binding force does not depend on express legislative recognition. Examination of the Iranian Code of Criminal Procedure shows that the components of proportionality are reflected as dispersed restrictions in provisions governing evidence gathering. Nevertheless, the duty to observe proportionality is not confined to instances in which the legislature has prescribed such restrictions. Those restrictions must be interpreted in light of the principle’s components. Where the law is silent, judicial authorities remain bound to determine the justified limits of evidentiary interference on the basis of proportionality. Absent such an interpretation, decisions concerning the scope and manner of interference are left to case-specific judicial discretion, risking interference and infringement of individual rights in evidence gathering.