Use of dictionaries is an integral part of language learning and teaching. With the availability of smartphones, EFL, ESL and ESP students can access and download a variety of specialized mobile dictionaries Apps (MDAs). Specialized MDAs are of special importance for students learning English for engineering, business, and computer science purposes. They contain thousands of essential specialize technical terms covering several sub-categories within the engineering, business and computer science fields. They are a good tool for enriching the students' specialized vocabulary and helping them learn faster and remember more in a shorter time. This article aims to show ESP instructors examples of engineering, computer science and business MDAs available in the Google Play or Apple Stores; how specialized MDAs can be located; criteria for selecting specialized MDAs; the lexicographical features of specialized MDA and instructional stages with MDAs. Instruction with MDAs goes through three stages: pre-task, task, and post-task phases. The instructor serves as a facilitator. She can help students find and download MDAs that meet their needs and purposes and match their proficiency level; gives pre-questions; and follows the students up to make sure they are making the best use of MDAs. The article concludes with some recommendations for ESP pedagogy with MDAs.