Abstract: Modular pack systems that may be used for backpacking, biking, hunting, fly fishing, or other indoor or outdoor activities are disclosed. In one embodiment, a system is adjustable to fit different user torso sizes and includes attachment structures that may removably secure various detachable packs, pouches, or containers for different items needed. The attachment structures are generally spaced apart evenly in groupings throughout various locations on shoulder straps, a torso structure, and/or waistband, providing for standardization of pouches for attachment to the harness. Certain embodiments may also include hydration and waist packs, fabric flex points on shoulder straps that provide more flexibility in fitting over the shoulders, an adjustable lumbar support system for additional back support for the wearer, and/or flaps on the waistband for securing excess belt webbing after adjustment by the wearer.