A hydrodynamic focussing and sample collection device configured to focus a stream of sample fluid in three dimensions, the device being formed using photolithography, lamination, 3-D printing, micro-moulding or injection moulding the device comprising: an array of sheath flow inlet channels (3, 5, 20a, 21a) distributed around a central sample flow inlet channel, each of said flow inlet channels having a respective outlet; an array of sheath outlet collection channels (13, 15, 26, 27) distributed radially around a central sample outlet collection channel, each of said outlet collection channels having a respective inlet; and a detection chamber defining a detection zone located between said outlets and said inlets, wherein: said sample flow inlet channel and said sample outlet collection channel are aligned and spaced-apart from one another across said detection chamber such that a stream of sample fluid exiting the sample flow inlet channel via its outlet may be directed across the detection chamber, through the detection zone, and into the sample outlet collection channel via its inlet; and each said sheath flow inlet channel (3, 5, 20a, 21a) is aligned with and spaced-apart from a respective said sheath outlet collection channel (13, 15, 26, 27) across said detection chamber such that a stream of sheath fluid exiting the sheath flow inlet channel (3, 5, 20a, 21a) via its outlet may be directed across the detection chamber and into the respective sheath outlet collection channel (13, 15, 26, 27) via its inlet; said sheath flow inlet channels (3, 5, 20a, 21a) being configured to direct respective sheath streams into contact with and parallel to the sample stream exiting the sample flow inlet channel outlet; wherein the sheath flow inlet channels comprise two stacked sheath flow inlet channels (3, 5) located on opposite sides of the sample flow inlet channel in a first direction, and the sheath flow inlet channels further comprise two side sheath flow channels (20a, 21a) located on opposite sides of the stacked sheath and sample flow inlet channels in a second direction, the second direction being perpendicular to the first direction and perpendicular to the sample stream, wherein the side sheath flow channels (20a, 21a) have a height in the first direction equal to, or greater than an overall combined height in the first direction of the stacked sheath and sample flow inlet channels (3, 4, 5). an array of sheath flow inlet channels (3, 5, 20a, 21a) distributed around a central sample flow inlet channel, each of said flow inlet channels having a respective outlet; an array of sheath outlet collection channels (13, 15, 26, 27) distributed radially around a central sample outlet collection channel, each of said outlet collection channels having a respective inlet; and a detection chamber defining a detection zone located between said outlets and said inlets, wherein: said sample flow inlet channel and said sample outlet collection channel are aligned and spaced-apart from one another across said detection chamber such that a stream of sample fluid exiting the sample flow inlet channel via its outlet may be directed across the detection chamber, through the detection zone, and into the sample outlet collection channel via its inlet; and each said sheath flow inlet channel (3, 5, 20a, 21a) is aligned with and spaced-apart from a respective said sheath outlet collection channel (13, 15, 26, 27) across said detection chamber such that a stream of sheath fluid exiting the sheath flow inlet channel (3, 5, 20a, 21a) via its outlet may be directed across the detection chamber and into the respective sheath outlet collection channel (13, 15, 26, 27) via its inlet; said sheath flow inlet channels (3, 5, 20a, 21a) being configured to direct respective sheath streams into contact with and parallel to the sample stream exiting the sample flow inlet channel outlet; wherein the sheath flow inlet channels comprise two stacked sheath flow inlet channels (3, 5) located on opposite sides of the sample flow inlet channel in a first direction, and the sheath flow inlet channels further comprise two side sheath flow channels (20a, 21a) located on opposite sides of the stacked sheath and sample flow inlet channels in a second direction, the second direction being perpendicular to the first direction and perpendicular to the sample stream, wherein the side sheath flow channels (20a, 21a) have a height in the first direction equal to, or greater than an overall combined height in the first direction of the stacked sheath and sample flow inlet channels (3, 4, 5).