Modern mobile phones, featuring high performance microprocessors, rich set of sensors and internet connectivity are largely diffused all over the world and are ideal devices for the development of low-cost sensing systems, in particular for low-income developing countries and rural areas that lack the access to diagnostic laboratories and expensive instrumentation. In the design of a smartphone based sensing system different elements must be taken in consideration such as sensors performance, acquisition rate and privacy preserving strategies when personal data must be shared in the cloud. In this paper a sensor-centric survey on smartphone based sensing systems is presented, covering different fields of application. Two different development approaches will be discussed: 1) the exploitation of the large number of sensors embedded in modern smartphones (high-resolution camera, microphone, accelerometer, gyroscope, magnetometer, GPS); 2) the interfacing with external sensors that communicate with the smartphone by the embedded wireless or wired communication technology.