Digital In-line Holographic Microscopy (DIHM) is a technique that provides depth and lateral resolution of the order of the wavelength throughout a volume of several cubic centimeters for visible light. This outstanding characteristic is reached by means of a simple optical setup and numerical reconstruction of the recorded holograms. It makes DIHM the right tool for applications in many microscopic studies. In this paper we study microfluidic phenomena by means of DIHM. To this end we seed a fluid with micron-size trackers (latex microspheres) and follow their displacement within an observation volume. We apply this technique to several situations such as the flow around a big sphere, flow through microchannels, bubbles in a fluid, bacterial motion in a diatom and the swimming behavior of paramecia and algae in water. By taking advantage in DIHM of the plane-to-plane reconstruction through a large depth of field, we generate 3D renderings of the paths followed by the trackers to produce a complete picture of the flow pattern, i.e. streamlines and velocity fields.