In this article we discuss multimodal technologies that address the technical and usability constraints of the mobile phone or PDA. These environments pose several additional challenges over general mobility solutions. This includes computational strength of the device, bandwidth constraints, and screen size restrictions. We outline the requirements of mobile multimodal solutions involving cellular phones. Drawing upon several trial deployments, we summarize the key designs points from both a technology and usability standpoint, and identify the outstanding problems in these designs. We also outline several future trends in how this technology is being deployed in various application scenarios, ranging from simple voice-activated search engines through to comprehensive mobile office applications.