The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX₂ (M = Mo, W; X = S, Se, Te) are investigated from first principles calculations. The band alignments between different MX₂ monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering.