During an investigation, police investigators collect physical and witness evidence, which can affect both investigators’ and suspects’ decision-making. This thesis examined the effects of evidence on the two parties. In Chapter 2 (Study Ⅰ), South Korean investigators (N = 202) read four crime reports concerning one suspect. The piece of critical evidence was manipulated and each report included one out of four evidence types (DNA, CCTV, fingerprint, and eyewitness). They then rated their perceptions of the suspect’s likelihood of commission and reliability of the critical evidence. The results showed that eyewitness testimony (vs. DNA, CCTV, fingerprint) significantly decreased investigators’ ratings. Chapter 3 (Study Ⅱ) was designed to examine the effects of evidence on interrogators’ selection of tactics when interrogating a suspect. Interrogators (N = 106) were randomly allocated to one of five homicide scenarios where only one type of critical evidence (DNA, CCTV, fingerprint, eyewitness, or no evidence) identified a suspect. Then, they were asked to choose tactics to interrogate the suspect. A list of 27 tactic names and descriptions was given for their selection, which was classified into five types of tactics: (a) Evidential/Reliable, (b) Evidential/Unreliable, (c) Nonevidential/Crime-Relevant, (d) Nonevidential/Crime-Irrelevant, (e) Context-Manipulation. Evidence type did not significantly affect the type of the chosen tactics. Chapter 4 (Study Ⅲa) was conducted with prisoners and suspects (N = 59) to examine how evidence perception would vary. Participants rated their evidence perception for the five tactic types. Participants tended to infer that the interrogator held more evidence when the Evidential/Reliable tactics were employed. Study Ⅲb surveyed laypersons (N = 117). The same design, procedure, and materials as in Study Ⅲa were adopted. Similarly, laypersons’ ratings were significantly higher for the Evidential/Reliable tactics than the other four types. However, prisoners’ and suspects’ ratings fluctuated much more across the 27 individual tactics than laypersons’. In summary, evidence appears to be influential on some of the investigators’ judgements. Also, some tactics may be influential on suspects’ perception of evidence.