A major component of every Safety Instrumented System (SIS) is the logic solver. It plays an important role as it is the sub-system which performs the logic in every Safety Instrumented Function (SIF). But sometimes there is confusion as to what constitutes: 1. Validation of the SIS logic solver. 2. Proof Test of the SIS logic solver. 3. Re-Validation of the logic solver. This paper will attempt to clarify the above with an explanation based on IEC 61508, 61511 (ISA84.00.01). It will answer the questions "When and How" for each of the activities listed.