A compact heat exchanger comprising: a plurality of layers (12A, 12B, 12C) affixed to one another and formed via additive manufacturing by placing successive layers of particles atop one another and selectively applying a form of concentrated energy to each layer of particles to fuse certain of the particles together and/or to another layer and together forming a core and a header apparatus; the core having formed therein a plurality of channels, the plurality of channels comprising a number of first channels and a number of second channels, at least a portion of the number of first channels being positioned for being in heat transfer relation with at least a portion of the number of second channels; the header apparatus comprising at least a first header that is in fluid communication with at least some of the channels of the plurality of channels; the at least first header having a channel end and a connection end, the channel end being situated adjacent the core and including a number of flow connections that are in direct fluid communication with the at least some of the channels, the connection end having an opening that is structured to be connected in fluid communication with another flow structure, the at least one header comprising a flow passage that extends between the channel end and the connection end and that enables fluid communication between the number of flow connections and the opening; and characterized in that at least one of the layers of the plurality of layers being a layer that comprises at least a portion of the core having formed therein at least a portion of a channel of the plurality of channels and that further comprises a portion of the at least first header that includes at least a portion of the opening and that has formed therein at least a portion of the flow passage; wherein the at least first header comprises a number of flow directors (54, 56), and wherein the flow passage comprises a plurality of flow passage portions (46, 48, 50) that together extend between the opening and a corresponding channel of the at least some of the channels and that permit fluid flow therebetween, at least a first flow director of the number of flow directors being situated adjacent the core and between a pair of first channels of the number of first channels, the at least first flow director having an external surface, a portion of the external surface forming at least a part of a flow passage portion of the plurality of flow passage portions that is in fluid communication with a first channel of the pair of first channels. a plurality of layers (12A, 12B, 12C) affixed to one another and formed via additive manufacturing by placing successive layers of particles atop one another and selectively applying a form of concentrated energy to each layer of particles to fuse certain of the particles together and/or to another layer and together forming a core and a header apparatus; the core having formed therein a plurality of channels, the plurality of channels comprising a number of first channels and a number of second channels, at least a portion of the number of first channels being positioned for being in heat transfer relation with at least a portion of the number of second channels; the header apparatus comprising at least a first header that is in fluid communication with at least some of the channels of the plurality of channels; the at least first header having a channel end and a connection end, the channel end being situated adjacent the core and including a number of flow connections that are in direct fluid communication with the at least some of the channels, the connection end having an opening that is structured to be connected in fluid communication with another flow structure, the at least one header comprising a flow passage that extends between the channel end and the connection end and that enables fluid communication between the number of flow connections and the opening; and characterized in that at least one of the layers of the plurality of layers being a layer that comprises at least a portion of the core having formed therein at least a portion of a channel of the plurality of channels and that further comprises a portion of the at least first header that includes at least a portion of the opening and that has formed therein at least a portion of the flow passage; wherein the at least first header comprises a number of flow directors (54, 56), and wherein the flow passage comprises a plurality of flow passage portions (46, 48, 50) that together extend between the opening and a corresponding channel of the at least some of the channels and that permit fluid flow therebetween, at least a first flow director of the number of flow directors being situated adjacent the core and between a pair of first channels of the number of first channels, the at least first flow director having an external surface, a portion of the external surface forming at least a part of a flow passage portion of the plurality of flow passage portions that is in fluid communication with a first channel of the pair of first channels.