The leaf essential oils of twelve species of Piper (Piperaceae) from Monteverde, Costa Rica (Piper aequale, Piper amalago, Piper biasperatum, Piper bredemeyeri, Piper dotanum, Piper fimbriulatum, Piper glabrescens, Piper imperiale, Piper lanceifolium, Piper nemorense, Piper oblanceolatum, and Piper sp. A near aereum) have been obtained by hydrodistillation and analyzed by GC-MS. The Piper leaf oils have been screened for antibacterial activity against Bacillus cereus, Staphylococcus aureus, and Escherichia coli; for Artemia salina (brine shrimp) lethality; for in-vitro cytotoxic activity against MCF-7 (human breast tumor) cells; and inhibition of the cysteine protease cruzain. A cluster analysis comparison with previously published Piper leaf oil compositions was carried out in order to discern the differences and similarities between the volatile chemical compositions of Piper species.