We investigate the behaviour of DNA-colloid systems using micro-rheology, with a view to demonstrating the efficacy of passive particle-tracking methodologies and developing entirely new systems. Chapter 1 introduces the fields of DNA coated colloids (DNACCs) and passive micro-rheology, with a particular fo cus on the challenges of creating an equilibrating DNACC system and the practicalities and limitations of passive microrheology in gaining access to valid rheological information. In Chapter 2, we present a newly developed realtime monitoring algorithm for complex moduli in optical tweezer micro-rheology sys,tems. Further to eliminating high frequency artefacts, our method is memory light and computationally efficient. Chapter 3 investigates the dynamics of ADNA coated colloids using Brownian Dynamics simulation and a theoretical model, also applying the algorithm developed in Chapter 2. A two-regime diffusivity is identified, in contrast to previous works, which simply found an increased hydrodynamic size. Chapter 4 looks at tuning the hydrophobicity of silica particles using poly(L)lysinepolyethylene glycol (PLL-PEG). We find an incubation pH dependence on their coverage. From analysing video microscopy trajectories, PLL-PEG coated beads sedimented onto A-DNA brushes are found to be significantly more diffusive. In Chapter 5, we int roduce an entirely new DNACC system, the functionalised fd bacteriophage, where high aspect ratio filamentous virions are coated with short oligonucleotides. Aggregation behaviour is confirmed with Atomic Force Microscopy and Dynamic Light Scattering, and systems where rods can act as a linker between spherical particles are also briefly investigated.