This chapter objective is to design an attitude estimation algorithm based on inertial and micro electro mechanical system (MEMS) sensors. It explores a wearable inertial and magnetic MEMS sensors assembly based on an inertial measurement unit composed of a 3-axis accelerometer, a 3-axis magnetometer and a 3-axis gyroscope. The application in mind is related to a free-ranging animal case in Bio-logging. By considering the rigid body kinematic model, a complementary filter is proposed in order to take advantage from the good short-term precision given by rate gyro integration and the reliable long-term accuracy provided by accelerometer and magnetometer measurements. The chapter follows: first section describes the problem statement and motivation for motion estimation in Bio-logging, second section details the attitude parameterization and the sensor measurement models used in this work, Third section details the structure of the proposed complementary filter for attitude estimation. Fourth section is devoted to experimental results and comparisons to illustrate the effectiveness of the proposed algorithm.