The remarkable discovery of cosmic acceleration poses two fundamental questions. (1) Does acceleration reflect the presence of a new energy component or the breakdown of General Relativity on cosmological scales ? (2) If acceleration is caused by a new energy component, is it constant in space and time as expected for fundamental vacuum energy, or does it show evolution or variation that imply a dynamical field ?
I will review observational methods for addressing these questions, with emphasis on baryon acoustic oscillations (BAO) and on galaxy clusters. I will show recent BAO results from the SDSS-III BOSS survey and recent cluster results based on cluster catalogs and stacked weak lensing mass measurements from SDSS-I and II. The methods being pioneered in these investigations will grow steadily more powerful with future surveys such as the Dark Energy Survey, BigBOSS, LSST, Euclid, and WFIRST.