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SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a programdedicated to the homogeneous and complete spectroscopic follow-up of X-ray AGNand galaxy clusters over a large area ($\sim$7500 deg$^2$) of the extragalacticsky. SPIDERS is part of the SDSS-IV project, together with the Extended BaryonOscillation Spectroscopic Survey (eBOSS) and the Time-Domain SpectroscopicSurvey (TDSS). This paper describes the largest project within SPIDERS beforethe launch of eROSITA: an optical spectroscopic survey of X-ray selected,massive ($\sim 10^{14}$ to $10^{15}~M_{\odot}$) galaxy clusters discovered inROSAT and XMM-Newton imaging. The immediate aim is to determine precise($\Delta_z \sim 0.001$) redshifts for 4,000-5,000 of these systems out to $z\sim 0.6$. The scientific goal of the program is precision cosmology, usingclusters as probes of large-scale structure in the expanding Universe. Wepresent the cluster samples, target selection algorithms and observationstrategies. We demonstrate the efficiency of selecting targets using acombination of SDSS imaging data, a robust red-sequence finder and a dedicatedprioritization scheme. We describe a set of algorithms and work-flow developedto collate spectra and assign cluster membership, and to deliver catalogues ofspectroscopically confirmed clusters. We discuss the relevance of line-of-sightvelocity dispersion estimators for the richer systems. We illustrate ourtechniques by constructing a catalogue of 230 spectroscopically validatedclusters ($0.031 < z < 0.658$), found in pilot observations. We discuss twopotential science applications of the SPIDERS sample: the study of the X-rayluminosity-velocity dispersion ($L_X-\sigma$) relation and the building ofstacked phase-space diagrams.

SPIDERS: the spectroscopic follow-up of X-ray selected clusters of galaxies in SDSS-IV