It depends. If your clutch has low miles on it (40,000 or less), chances are the slippage is due to one of two things: oil contamination or a misadjusted clutch linkage. If your clutch has a lot of miles on it (60,000 or more), chances are it's worn out and you need to replace it.
To rule out oil contamination as a possible cause of slippage, check under the rear of the engine and the bellhousing for oil leaks. If you see oil on the oil pan or bellhousing, the rear main oil seal is probably leaking. Other leak points include manifold and valve cover gaskets at the back of the engine, and the transmission input shaft seal.
If you've got an oil leak, don't replace the clutch until you've fixed the leak. Once the clutch linings have been contaminated by oil, there's no way to clean them. Replacing the clutch disk is the only way to restore proper clutch operation.
If you don't have a leak, check the linkage adjustment. Most cars with a cable linkage have an automatic adjusting mechanism that's supposed to maintain proper clearances. If anything, the cable would be too loose rather than too tight. But if someone has been playing around with the linkage adjustment, they may have gotten it too tight. The same goes for vehicles with hydraulic linkages. There's no way this type of linkage can cause slippage unless it is misadjusted by someone.
That leaves the clutch itself. Slippage can be caused by two things: worn facings or loss of spring tension in the pressure plate. Unless the clutch really has been abused or has a lot of miles on it, it's unlikely the pressure plate is weak. Normal wear reduces the thickness of the facings on the clutch disk, which in turn reduces the clamping force the pressure plate can apply to squeeze the disk against the flywheel. Replacing the clutch disk should cure the problem.
Even so, the clutch and flywheel should be carefully inspected when the parts are removed. If the pressure plate is worn or damaged, you'll need to replace that, too. Most experts recommend having the flywheel refaced to restore the friction surface. You can probably get by without refacing the flywheel -- but only if the flywheel is flat, smooth, clean and uncracked. Any grooves, heat discoloration, cracking or other damage would call for resurfacing or replacing the flywheel.