Abstract: The paper reports on research into how effectiveness of military systems may be calculated, in the light of recent advances in systems science and simulation methods. An alternative approach to conventional methods for calculating effectiveness is proposed. This "white-box" approach envisages a combat simulation where C3I technology, human behaviours and decision-making processes are explicitly represented, including (most importantly) their many, mutual interactions. The simulation further envisages two combatants, and measures the effect that each has on the other; hence, measures their comparative effectiveness. It is possible to identify putative emergent properties, capabilities and behaviours of combatants in given scenarios and environments. It is also possible to adjust the (simulated) performance of pieces of technology, individually and in sets. This enables the contribution of various equipments to overall effectiveness to be both observed and optimised. Hence, it is potentially possible to identify what the performance of each equipment should be, to maximise overall C3I effectiveness. In particular, the research was interested in the impact of COTS on C3I effectiveness within a naval domain. This was assessed by representing both combatants as having identical, bespoke technological systems, operated by identically trained and experienced personnel. Identical effectiveness then emerges from each combatant. Holding one combatant steady as a dynamic reference, a single item of equipment was changed in the other combatant from bespoke to COTS. Any subsequent change in effectiveness was attributable exclusively to that one change. It was also possible to explore the effects of continual upgrades to COTS over extended periods. Using this white-box approach, it seems possible to approach procurement by identifying the performance and other characteristics of equipments as they contribute synergistically to overall C3I effectiveness. This leads to performance measures for equipments being seen, not as individual quantities, but as interactive contributors to overall effectiveness. Since the simulation also represents costs of maintenance and support of technology, bespoke or COTS, it is also possible to determine the overall value of COTS versus bespoke on a scientific basis. The approach also enables radical tradeoffs to be explored. For instance, it may be possible to trade-off the cost of command team training against the cost of enhanced weapons.