The advent of mobile technology has brought computing to a wide range of new contexts, some of which are highly sensitive and place new constraints on the designer. In this chapter we discuss issues related to the design and evaluation of mobile software for sensitive situations, where access to the end user is extremely restricted. We focus on the specific example of technological interventions that support adolescents in mental health care settings. We examine the practical and ethical constraints placed on access to end users and contexts of use, and how this may affect approaches to design and evaluation. General design recommendations for this area are described. We consider approaches to iterative design with mental health care professionals, and how research on technological and therapeutic aspects may proceed in tandem. We identify methods that can be used when conducting evaluation in these limited situations and describe a methodology for maximising the value of such evaluation. By way of illustration, we present the design and evaluation of a mobile phone-based “mood diary” application designed for use in clinical situations by adolescents undergoing mental health interventions.