This work is concerned with the Sunyaev-Zel'dovich (S-Z) effect in clusters of galaxies and its use in cosmology. In Chapter 2, I consider possible causes of error in the determination of H₀ from the uncertain geometry of clusters and complications such as lack of virialisation. From numerical simulations, I find that with 20 clusters, the resultant error is 8%. Chapter 3 details and tests a new maximum likelihood algorithm for the removal of contaminating radio sources. Two new estimates of H₀ are presented, 78⁺²⁶_-20 dm s^-1 Mpc^-1 for 0016+16 and 40⁺¹⁴_-11 km s^-1 Mpc^-1 for A611. In chapters 4 and 5 I describe a search for new clusters via extended steep-spectrum radio emission due to merger events or head-tail sources. A candidate sample of possible high-redshift clusters is produced based on S_326MHz > 65 mJy and α^1.4GHz_0.3GHz > 1.5. Six of these were then used as targets for S-Z observations; no unequivocal detection was found implying each cluster has a total mass ≲ 10¹⁵ M_⊙. Optical V, R_S and I follow-up in two of the fields shows two galaxies in each field with V - R_S ≈ 3 and R_S ≈ 20; similar observations of blank fields show no such galaxies and hence these are good candidate cluster members at z = 0.5-0.6.