RNA has important and diverse roles in biology, but molecular tools to manipulate and measure it are limited. For example, RNA interference(1-3) can efficiently knockdown RNAs, but it is prone to off-target effects(4), and visualizing RNAs typically relies on the introduction of exogenous tags(5). Here we demonstrate that the class 2 type VI6,7 RNA-guided RNA-targeting CRISPR-Cas effector Ca.s13a(8) (previously known as C2c2) can be engineered for mammalian cell RNA knockdown and binding. After initial screening of 15 orthologues, we identified Cas13a from Leptotrichia wadei (LwaCasl3a) as the most effective in an interference assay in Escherichia coli. LwaCasl3a can be heterologously expressed in mammalian and plant cells for targeted knockdown of either reporter or endogenous transcripts with comparable levels of knockdown as RNA interference and improved specificity. Catalytically inactive LwaCasl3a maintains targeted RNA binding activity, which we leveraged for programmable tracking of transcripts in live cells. Our results establish CRISPR-Cas13a as a flexible platform for studying RNA in mammalian cells and therapeutic development.